Summary As the environment is a crucial source in an infant's development, it is important to assess the proximal environment where a variety of social relationships take place. Experiences derived from the home environment allow the specific activities that a child builds actively. These opportunities have an outstanding impact on an infant's development. The home concept and its influence on development led to the discovery and interrelations of several dimensions and today's emphasis relies on identifying the mechanisms of the variability of environment that exert an influence on the variability of development. Under this perspective, at the end of the 1960's the Infant/Toddler HOME Inventory was developed by Caldwell and Bradley. Very soon, Cravioto adapted it for its use in a Mexican population according to the characteristics of typical families and cultural aspects. The purpose of this investigation was to establish the internal consistency of the Infant/Toddler HOME Inventory in the version adapted by Cravioto through the assessment of the precision of its scoring in terms of internal consistency. Cohort 62 infants and their mothers were contacted in the first 72 hours after delivery through a Research Program from the Child Psychiatry Hospital Dr. Juan N. Navarro (Environmental Modulation of Infancy Development). They were born in two Mexico City hospitals a state public hospital, Hospital de la Mujer (Women's Hospital) and in the Mother-infant Research Center from the Birth Study Group (CIMIGEN). All of them were low-risk infants who met the criteria for a one year follow up and whose parents gave their informed consent and accepted to be visited at their homes. The distribution of the infants group by sex was 30 infants (48%) were female and 32 (52%) were male. Families were classified as follows 72.5% were nuclear and 27.5% were extended. According to their socioeconomic profile, a high relationship was found between income and housing seven (11.3%) of the highest income families lived in houses or apartments of their own or rented, where there were more rooms than inhabitants, whereas all others were living as follows crowded houses or apartments (23 families, 37.1%), houses with of only one room (4 families, 6.5%) and the poorest houses with collective bathroom (28 families, 45.2%). All households had electrical lighting and most of them had also drinkable water inside their homes (98.4%); one shared water from a deep pond with their community. Bathrooms were 35.5% with running water, 59.7% used a bucket for carrying water and 4.8% had latrine. Instrument The Infant/Toddler HOME Inventory, version adapted by Cravioto, with 62 items in a binary format response was used. Subscale VII, Play Materials, asked about color, size, consistency or texture and type of toys. The instrument is administered by having a person calling the home at a time when the infant is awake and can be observed interacting with the mother or principal caregiver. The internal consistency of the Infant/Toddler HOME Inventory was monthly assessed for each of its subscales and the total scale, until the child's first year of life (12 months), applying Cronbach's alpha. Results showed that alpha coefficient's values higher than 0.60 throughout the 12 months were observed in the VII Play Materials shoed a range of 0.64 and 0.84, and 0.60 and 0.83 for the Total Scale. On the other hand, subscale VI, Physical Environment, showed a less than 0.60 value with an internal consistency coefficient of 0.56 in the first month of life; nevertheless the remaining months had values between 0.60 and 0.70. This was also observed with subscale II, Mental Development and Vocal Stimulation, with a 0.58 value for the first month of life and alpha values between 0.63 and 0.74 for the remaining eleven assessments. Subscale III, Emotional Climate, exhibited seven evaluations in a range between 0.61 and 0.76; this is the second with a 0.61, fourth with 0.69, sixth with 0.76, seventh with 0.67, ninth with 0.63, tenth with 0.69 and eleventh 0.63. Non-acceptable internal consistency, it is less than 0.60 in most of their scorings were observed for the following scales I. Adult Contact, V. Breadth of Experience. Only in the sixth month's a value of 0.64 was observed, and the rest of scales exhibited a range within a 0.38 as inferior limit and 0.56 superior limit in the former and a 0.65 value in the tenth month and 0.60 in the eleventh. Subscale IV, Avoidance of Restriction, showed the lowest coefficient with values between 0.24 and 0.49. These results suggest that most of the subscales had adequate reliabilities, except for subscales I, Adult Contact; IV, Avoidance of Restriction; and V, Breadth of Experience. The purpose of this study was to evaluate the internal consistency of an instrument which measures the home environment quality in infants during their first year of life. Results showed the Infant/Toddler HOME Inventory had high internal consistency values in the Total Scale. These results are similar to those obtained by Banard, Bee & Hammond with a group of 179 children in Seattle, where they found 0.77 values at fourth months, 0.81 at eight and 0.86 at twelve. Our findings were within a rank of 0.68 and 0.83. Cronbach's alpha value obtained for the different subscales showed intersubscale differences. Subscales II, III, VI and VII showed internal consistency values equal or higher than 0.60 in most of the assessments during the 12 months of life. This implies measurements are precise and reliable when using them in low risk Mexican infants. Lower values found in subscales I, Adult Contact, IV, Avoidance of Restriction and V, Breadth of the Experience, may be due to the low number of items. According to Nunnally and Bernstein, it is recommended that measurements with a low internal consistency should be used with initial, non-crucial decisions, and with temporary and reversible effects susceptible to replication and rectification. It is very important to stress the fact that this Infant/Toddler HOME Inventory version is not comparable with other in the literature because the structure was notably modified. Limitations in this study are not only this modification, but the size sample, and future research efforts should overcome this fact by trying to sample infants from different Mexican regions. This study points out the need to adapt, develop and evaluate psychometrically instruments that measure specific aspects of the environment of infant's homes. Our results may be an initial step for those interested in measurements of Mexican families homes, or in those interested in the Infant/Toddler HOME Inventory as an indicator of the environmental aspects in early infancy.