Leptin, a 16-kDa protein, has proinflammatory properties and has been linked to respiratory physiological responses in majority white populations. Little is known, however, about the relationship of leptin with lung function in nonwhites. Cross-sectional associations of circulating serum leptin concentrations with forced expiratory volume in 1 s (FEV1), FEV in 6 s (FEV6), and vital capacity (FVC), assessed by spirometry, were examined in 4,679 African-American men and women participants (54.3 +/- 12.4 years; 62.7% women) in the Jackson Heart Study (JHS). The independent association of leptin was examined in relation to FEV1, FEV6, and FVC% predicted after adjustment for age, education, smoking status, pack-years of cigarette smoking, respiratory medication use, and menopausal status in women; additional adjustment included total body weight, waist circumference, and BMI. Serum leptin was inversely related to FEV1, FEV6, and FVC% predicted values in men. A dose-response relationship was observed with men in the highest leptin quartile having a significantly lower lung function compared to men in the lower leptin quartile. BMI significantly modified this relationship in women: leptin was most consistently associated with lung function in obese women, less consistent in overweight women, and absent in normal-weight women. Serum leptin concentration was strongly, inversely, and independently associated with lung function in African Americans, especially African-American men and obese women.