BACKGROUND Acne is the most common symptom prompting patients to consult a dermatologist. No previous study has been conducted in France to determine the prevalence of acne and describe the main epidemiological features. SUBJECTS AND METHODS A cross sectional study was conducted in November 1996 and included 913 school children aged 11 to 18 years. This sample was statistically representative of the entire secondary school population in France during the 1996-1997 school year. The subjects were stratified by 5 criteria: age, sex, rural or urban residence, sun exposure, type of school. RESULTS Taking the clinical diagnosis made by the dermatologist investigator as the main criteria, the overall prevalence of acne was 72 p. 100. It was 76.1 p. 100 using the new ECLA grading system previously described. The prevalence of acne was sex and age dependent: highest scores were found for girls aged 14-16 years and for boys aged 16-17 years. Genetic factors were very important for the outcome of acne. Finally, 41 p. 100 of the acneic subjects were following a treatment, prescribed by a dermatologist in two-third of the cases. DISCUSSION These results are in agreement with those previously published in the literature although some differences were disclosed. It would appear important to distinguish between minimal acne with a few retentional pimples occuring during adolescence and severe acne (more than 20 pimples on the face) requiring early medical care to avoid scarring.