To determine the prevalence of asthma and to examine the pattern of health service utilization of asthmatic children in Baltimore, we sent questionnaires randomly to 4096 first and sixth graders attending Baltimore City public schools; 2898 completed the questionnaire. Asthma was defined as "a condition which causes difficulty in breathing, with wheezing noises in the chest." On the basis of this definition, we found that the cumulative prevalence of asthma was 10.5% and the 12 mo period prevalence was 7.2%. The prevalence was significantly higher for boys (male:female = 1.6:1) and for blacks (black:white = 1.5:1). Nearly 50% of the children with active asthma missed 6 days or more out of the school year because of illnesses due to asthma. Almost half the asthmatic children obtained their care of asthma in the emergency room, and twice as many blacks as whites used the emergency room as their primary source of care. Moreover, emergency room users had a higher school absentee rate than non-emergency room users.