To identify the main determinants of health services utilization by children under five years of age, a population-based study using systematic sampling was conducted in Rio Grande, Southern Brazil. Twelve previously trained interviewers applied a standardized home questionnaire to mothers in order to obtain data on living conditions, medical consultations, and hospitalization for children under five years of age. Among 514 children covered by the study, 50% had required consultation with a physician in the previous three months and 11% had been hospitalized in the previous twelve months. Acute respiratory infection was responsible for almost two-thirds of the consultations and half of the hospitalizations. After adjusting the analysis for several confounders, the most important determinants were children's age, father's schooling, and type of home construction. Identification of these factors can contribute to adequate planning of future health interventions and to reach children in the community who need but have not received health care.