The prevalence and severity of asthma appears to be greater in blacks than in whites. To determine if racial differences in airway responsiveness may explain these findings, methacholine challenge tests from 62 black and 238 white women 20 to 35 yr of age were evaluated. Subjects served as controls for a case-control study of the relation of airway responsiveness and preterm labor. Standardized questionnaires were used to obtain information on age, obstetrical history, education, income, cigarette smoking, medication use, and respiratory illnesses and symptoms. Total serum IgE was measured using a radioimmunoassay. Methacholine challenge testing was performed on all subjects 6 wk after delivery, and the provocative dose causing a 20% decrease in FEV1 (PD20) was calculated. Black women in the study had more pregnancies and children, were younger, less well educated and more impoverished, and reported greater cigarette smoking and less medication use than did the white women. Additionally, black women had higher geometric mean serum IgE levels (blacks: 65.4 IU versus whites: 20.0 IU; p < 0.001), lower FEV1 (blacks: 2.73 +/- 0.38 SD L versus whites: 3.19 +/- 0.39 L; p < 0.001), and greater unadjusted airway responsiveness than did white women (geometric mean PD20: blacks: 28.4 mumol versus whites: 38.8 mumol; p = 0.02). After adjusting for selective demographic and smoking differences, a significant additional effect of race on mean PD20 was found. However, after adjustment for level of serum IgE and level of FEV1, racial differences were no longer apparent.(ABSTRACT TRUNCATED AT 250 WORDS)