We examined the relationship between asthma prevalence and BMI in a cross-sectional survey of 471,969 adolescents. The size of the survey allowed us to investigate this relationship with much greater resolution than previously possible. Both lifetime and current asthma prevalence increased monotonically with increasing BMI, starting with individuals as low as the 45th to 55th percentiles of BMI. The pattern was similar between males and females and among six racial/ethnic groups. The results suggest that weight reduction even among persons not classified as overweight or obese may be an important component of asthma management.