We investigated ethnic differences in spirometry and gas transfer (DL(CO)) in a young, healthy population of nonsmoking physicians and medical students aged 22-33 yr, of European or Asian descent. Each answered questions detailing ethnic background, medical history, level of physical activity, and length of residence in the United States. Spirometry and single-breath DL(CO) maneuvers were performed in accordance with ATS standards. Venous blood was measured for hemoglobin (Hb). The same equipment was used to test all subjects. Data were analyzed by multiple linear regression. Eighty subjects were studied, with 20 in each of the following groups: European male, European female, Asian male, and Asian female. Asian values for forced vital capacity, forced expiratory volume in 1 s (FEV(1)), and alveolar volume (VA') were significantly lower than for Europeans, but DL(CO), DL(CO)/VA', and DL(CO)/VA'/Hb did not differ significantly. These differences could not be attributed to age, length of residence in the United States, activity level, or variance in baseline characteristics and anthropometric measurements, and therefore represent a true physiologic difference. Ethnic differences between individuals of Asian and European backgrounds should be considered when interpreting pulmonary function tests, especially when predicted values are based on populations of European descent.