BACKGROUND: Ambulatory Care Sensitive Conditions (ACSC) are a set of selected codes of hospital discharge diagnosis intended to measure avoidable hospitalizations. Primary Health Care Services may avoid the hospitalizations due to ACSC by applying any of the following interventions, that are characteristics of this level of care: a) primary prevention; b) secondary prevention, and c) tertiary prevention and rehabilitation. METHODS: Observational, cross-sectional pilot study on 248,174 hospital discharges, from a population of 2,248,704 inhabitants in 161 Basic Health Care Units (BHCU), recorded in the minimum basic set of hospital discharge data in Catalonia during 1996. The complete list of diagnostic codes of ACSC, identified in the literature search, has been used. Crude and age-standardised hospitalisation rates have been estimated. Standardised hospitalisation ratio (SHR) has been computed to compare hospitalisation rates between BHCU. To analyse the effect of variables associated with high ACSC admission rates, multivariate analysis has been carried out by means of Poisson's regression. RESULTS: Hospitalisation due to ACSC account for 13% of all hospitalizations and 16% of hospital stay days. Acute and chronic diseases of the lower respiratory tract are the first cause of hospitalisation at any age group (< 5, 15-64 and > or = 65 years). The most common diagnostic groups, for all ages are respiratory disorders (acute and chronic diseases of the lower respiratory tract and pneumonia) and urinary tract conditions (pyelonephritis/urinary tract infection). The so-called self-limited health problems (ear, nose and throat mild infections/infections of the upper respiratory tract, febrile convulsions in children and gastroenteritis) account for more than 10% of all hospitalizations by ACSC, 45% of which occur in children. The overall crude hospitalisation rate by ACSC is 146.9/10,000 inhabitants (range: 12.4/10,000-239.9/10,000). SHR ranges from 0.01 to 1.85. CONCLUSIONS: "Hospital admissions due to ACSC" is a valid indicator to assess global performance of Primary Health Care and it identifies a part of hospital activity that is amenable to be cared for at Primary Health Care level and therefore potentially avoidable.