'Conventional' (lung volumes, airway conductance, CO transfer factor) and 'small-airway dysfunction' (single-breath N2 washout, maximal expiratory flows with air and a helium-oxygen mixture) tests were performed in a group of asymptomatic male ex-smokers (n = 20) with normal spirometry and compared to those of a similar group of middle-aged nonsmokers (n = 41). The study group had a mean life-long consumption of 13.2 +/- (SD) 8.8 pack-years and had stopped smoking 1-21 years ago, mean 7.3 +/- (SD) 5.8 years. Among the conventional tests, a minimal increase in residual volume and residual volume/total lung capacity ratio was found. Ex-smokers had a steeper phase III slope, a higher closing volume/vital capacity ratio, lower forced flows with air and a higher volume of isoflow, but neither difference between the group means was significant. There was a trend to more abnormal small-airway tests as a function of increased smoking, but only the correlation between percent of predicted closing volume and pack-years reached significance (r = +0.51, p less than 0.05). We conclude that small-airway function normalizes in the long term in most ex-smokers if they become asymptomatic, with less than 10% still having abnormal results years after they stopped smoking.