This paper reports rates of psychopathology in a population of 9- and 13-yr olds from a Spanish slum. Two cohorts of all the children born in 1989 and in 1993 and registered in the census of a municipality in 2001 were assessed over a 3-yr period with structured diagnostic interviews and functional measures. In the first year of the study 79 (53.7%) children of the adolescent 13-yr-old population and 72 (59.5%) of the pre-adolescent 9-yr-old population participated. Between 30% and 60% of preadolescents and between 30% and 50% of adolescents presented some mental disorder. Anxiety and disruptive behavior disorders were the most frequent disorders in both cohorts. For both genders, the highest risk for any psychopathology was at 10 yr. We found that, psychopathology and functional impairment decreased with age, and that the psychopathology of children in a peripheral slum of a big city is 3 times higher than the median of the general population. This information should be useful for administrators providing services for children from the most disadvantaged segment of the population.