Racial/ethnic and socioeconomic disparities in the prevalence of childhood asthma have been reported nationally but few population-based studies in local and regional settings have been reported. To assess variation in the prevalence of childhood asthma and associated morbidity across race/ethnic and income groups in the Los Angeles County population, we analyzed data on a random sample of 6004 children (< or = 17 years old) enrolled in a countywide health survey from September 1999 through April 2000. The prevalence of childhood asthma was highest in blacks (15.8%), intermediate in whites (7.3%) and Asians (6.0%), and lowest in Latinos (3.9%; p < 0.001). These differences persisted after controlling for income, measures of health care access, and other covariates. Asthma prevalence was inversely related to income in all racial/ethnic groups except Latinos from Spanish-speaking households. Among children with asthma, blacks and Latinos were more likely than whites to report asthma-related limitations in physical activity and need for urgent medical services. These findings indicate marked disparities in asthma prevalence and related morbidity in this large urban child population and highlight the importance of efforts to identify high-risk subpopulations for focused prevention and treatment interventions.