OBJECTIVE: Ambulatory care-sensitive conditions (ACSC) are health problems managed by actions at the first level of care. The need for hospitalization by these causes is avoidable through an effective and proper primary health care. The objective of the study was to estimate ACSC among patients hospitalized by the Sistema Unico de Saúde (Brazilian Health System). METHODS: Hospital-based cross-sectional study involving 1,200 inhabitants of Bagé (Southern Brazil) who were inpatients between September/2006 and January/2007. The patients answered a questionnaire applied by interviewers and were classified according to the model of attention utilized prior to hospitalization. ACSC were defined in a workshop promoted by the Ministry of Health. The variables analyzed included demographic and socioeconomic characteristics, health and health services utilized. Multivariate analysis was conducted by the Poisson model, according to a hierarchical conceptual framework, stratified by sex and model of care. RESULTS: ACSC accounted for 42.6% of the hospitalizations. The probability that the main diagnosis for hospitalization is considered an ACSC is greater among women, children under five years of age, individuals with less then five years of schooling, hospitalization in the year prior to the interview, emergency room consultation, and being an inpatient at the university hospital. Among women, ACSC are associated with age, educational level, length of time the health center has been in existence, living in an area covered by the Programa Saúde da Família (Family Health Program), use of this service, emergency room consultation during the month prior to the interview and hospital to which patient was admitted. For men, it was associated with age, have undergone another hospitalization in the year prior to the interview and hospital to which patient was admitted. CONCLUSIONS: Analysis of ACSC allows identifying groups with inadequate access to primary health care. Although we could not infer an effect on the risk of hospital admission, analysis by sex and model of care suggests that Family Health Program is more equitable than "traditional" primary health care.