The prevalence of epilepsy among a sample of children born in 1966 and investigated when they were 9 years old, was studied in Tlalpan, a sector of Mexico City. The prevalence ratios for epilepsy per 1,000 population were: minimum 18.3, average 42.2 and maximum 44.3. The results are compared with those of three studies in other parts of the Americas, using the same methods. The positive predictive value of the test used to identify epileptics was nearly 20% and its possible use in mass screening programs is discussed. Prevalence ratios for simple febrile convulsions and minimal brain dysfunction are also reported.