Men with chronic obstructive pulmonary disease have reduced endogenous testosterone levels. Little is known, however, about the relationship between pulmonary function and endogenous testosterone levels in a general population. In the present study we have examined the cross-sectional associations between sex hormones measured by immunoassay and pulmonary function assessed with spirometry and oxygen saturation in 2,197 men participating in the fifth Tromsø study. The data were analyzed by univariate correlations, multiple linear regression analyses and analyses of variance and covariance. Total and free testosterone were positively and independently associated with forced vitality capacity, FVC (% of predicted) (P = 0.001 and P = 0.006, respectively) and forced expiratory volume in 1 second, FEV(1 )(% predicted) (P = 0.033 and P = 0.002, respectively), and men with severe pulmonary obstruction (FEV(1) % of predicted < 50) had lower free testosterone levels (P = 0.005). In this cross-sectional data from Tromsø, a reduction in pulmonary function was associated with lower levels of total and free testosterone. We suggest that the reduction of total and free testosterone could be due to an alteration of the hypothalamic-pituitary response.