The relationship of the peripheral leukocyte count to pulmonary function was examined in 1,510 men. There were 466 current cigarette smokers, 485 former cigarette smokers, and 559 never smokers enrolled in a longitudinal study of aging. Spirometry was performed twice on all subjects ten years apart. A multiple regression analysis indicated that leukocyte count was inversely related to baseline levels of both forced vital capacity (FVC) (p less than 0.001) and forced expiratory volume in one second (FEV1) (p less than 0.001) after adjustment for age, height, and smoking habits. Similarly, change in leukocyte count over a ten-year period was inversely related to follow-up levels of both FVC (p = 0.036) and FEV1 (p = 0.004) after adjustment for baseline levels of leukocyte count, pulmonary function, age, height, and smoking habits. These data suggest that the peripheral leukocyte count is an important determinant of level of pulmonary function. Further investigation of the pathophysiologic mechanisms involved in this association seems indicated.