The prevalence of asthma in the United States is higher than in many other countries in the world. Asthma, the most common chronic disease of childhood in the United States, disproportionately burdens many socioeconomically disadvantaged urban communities. In this review we discuss hypotheses for between-country disparities in asthma prevalence, including differences in "hygiene" (e.g., family size, use of day care, early-life respiratory infection exposures, endotoxin and other farm-related exposures, microbial colonization of the infant bowel, exposure to parasites, and exposure to large domestic animal sources of allergen), diet, traffic pollution, and cigarette smoking. We present data on socioeconomic and ethnic disparities in asthma prevalence and morbidity in the United States and discuss environmental factors contributing to asthma disparities (e.g., housing conditions, indoor environmental exposures including allergens, traffic air pollution, disparities in treatment and access to care, and cigarette smoking). We discuss environmental influences on somatic growth (low birth weight, prematurity, and obesity) and their relevance to asthma disparities. The relevance of the hygiene hypothesis to the U.S. urban situation is reviewed. Finally, we discuss community-level factors contributing to asthma disparities.