A survey was conducted in Merida, Yucatan, between February and June 1994 in order to determine the prevalence of asthma and the frequency of a family history of the disease in school-age children. A random sample was taken of 505 children of both sexes between the ages of 6 and 12 who were enrolled in elementary schools in the urban area of Merida. Every child's father or mother was surveyed and asked if the child had had recurrent episodes of loud, difficult breathing with wheezing chest sounds, if he or she had had bronchitis, asthmatiform bronchitis, or asthma, and if the mother, father, or any of the siblings had ever suffered from asthma. A total of 61 children with asthma (cumulative prevalence, 12%) was found. Among these children a history of maternal or paternal asthma was more frequent (25 and 18%, respectively) than in other children (10 and 6%, respectively), the difference being statistically significant (P < 0.002) in both cases. Results show the high prevalence of asthma in Merida and suggest that heredity plays a role in the development of the disease.