This study seeks to identify the incidence of unfavorable birth weight (UBW) -< 3000 grams, the factors associated with this condition, and the probability that a child has an UBW in the presence or absence of identified risk factors, in Tlaquepaque and Tonal , outlying areas of Greater Metropolitan Guadalajara, Mexico. A sample of live-born infants in 1991, children of mothers covered by the Mexican Institute of Social Security were selected from the study area; through multistage probabilistic sampling, random selection was made of Family Medical Units within the study areas, and of physicians' offices within those selected units; finally, all the liveborn infants in 1991 from these selected physicians' offices were studied: a total of 141 newborns were studied Mothers of the newborns chosen were interviewed; a questionnaire with different biomedical, socioeconomic, and demographic items was applied by social workers specially trained for the purpose. Logistic regression models were used lo estimate odds ratios (OR), with 95% confidence intervals (CI). The probability that a child would have UBW in the presence or absence of identified risk factors were also calculated. The results show that 22% of the infants studied were born weighing less than 3000 grams; four risk factors were statistically associated with UBW: mother's age of 35 or older (OR=18.47, CL 1.86-83.54); mother worked outside the home (OR= 3.14, C1:1.15-8.59); mother's pre-pregnancy low weight (OR= 5.04, CL1.04-24.47); and late detection of pregnancy (OR=2.64, CI: 1.02-6.84). In the presence of all the risk factors identified, there is a very high probability (0.97) that a child be born with birthweight less than 3000 g, and in the absence of these factors the probability is reduced substantially (0.04). The findings indicate the magnitude of the problem studied, but also the possibility of health services acting in a timely fashion, since the identified risk factors make it possible lo predict, with relative certainty, the birth of a child weighing less than 3000 g.