OBJECTIVE To find the prevalence of caries in the 12-year-old school population of the city of Barcelona. METHODS An epidemiological crossover survey of dental caries in a representative sample of children in Barcelona. METHODS 739 children in the 7th year of basic education, who were twelve years old at the moment of the examination and came from 35 schools in the city of Barcelona, were examined. RESULTS Prevalence of caries was 52% (60% in girls and 45% in boys) and the CAOD index (teeth with caries, absent because of caries or permanent teeth with fillings) was 1.37 (1.70 for girls and 1.12 for boys). 51% of the permanent teeth affected by caries had been filled. The distribution of the CAOD index showed that 23% (almost a quarter of the population studied) of 12-year old schoolchildren in Barcelona had 3 teeth or more affected by caries. In the logistical regression analysis, gender (being a girl, OR = 1.69) and the absence of a regular programme of fluoride mouthwashes (OR = 1.69) were associated with having a higher caries index (CAOD equal to or above 3). CONCLUSIONS The results show caries indices which are generally low and a tendency to stability. The programmes of fluoride mouthwashes seem to be effective in decreasing the incidence of caries.