To determine the prevalence of self-medication in children and adolescents in the municipalities of Limeira and Piracicaba, state of São Paulo, and to correlate results with sociodemographic indicators and with the use of health care services (public or private). Descriptive population-based study of a simple random sample from the two municipalities, comprised of 772 inhabitants from 85 urban census sectors selected through cluster sampling. age < or = 18 years; interview with one parent/tutor; consumption of at least one drug in the previous 15 days. Subjects were divided into two study groups according to their pattern of drug use: self-medication (lay advice) and medical prescription. Linear association tests, descriptive analysis of variables and multiple logistic regression tests were carried out to analyze data. The prevalence of self-medication was 56.6%. Mothers (51%) and drugstore employees (20.1%) were most frequently responsible for self-medication. The main groups of self-prescribed drugs were: analgesic/antipyretic and non-hormonal anti-inflammatory drugs (52.9%); drugs acting on the respiratory tract (15.4%) and gastrointestinal drugs (9.6%); and systemic antibiotics (8.6%). The situation that most commonly motivated self-medication were respiratory diseases (17.2%), fever (15%), and headache (14%). Subjects in the age group of 7-18 years (odds ratio = 2.81) and public health care users (odds ratio = 1.52) showed increased risk for self-medication. The prevalence of self-medication in children and adolescents was high, which reinforces the need for public health interventions aiming at preventing this practice.83453-8