To estimate adult asthma prevalence in the world's most rapidly growing mega-city, we applied epidemiologic surveillance tools, as a cooperating center of the European Community Respiratory Health Survey, to a randomly selected sample of Mumbai (Bombay) residents in 1992 through 1995. From a metropolitan population of over 10 million, we took a one-in-ten random sample from electoral rolls in a socially diverse residential district, and examined asthma symptoms in adults age 20 to 44 yr. In Phase I, we interviewed 2,313 adults about symptoms, asthma diagnosis, and medications in the previous 12 mo. In Phase II, family and smoking history, socioeconomic data, housing characteristics, serum IgE, allergy skin tests, spirometry, and methacholine challenge tests were obtained in a subset of 20% of those who had completed Phase I. House dust mite was the most common positive skin test (18% prevalence) and the only one of the nine applied that was significantly associated with asthma symptoms and physician-diagnosed asthma. Asthma prevalence was 3.5% by physician diagnosis, and 17% using a very broad definition including those with asymptomatic bronchial hyperreactivity. Asthma prevalence was strongly associated with positive house dust mite skin test, family history of asthma, and total IgE.