Non smoking, male professional firemen-divers (n = 20) underwent two pulmonary function tests (PFT) separated by 8-9 years. Measured data were compared to European Coal Steel Community recommended reference values to permit cross-sectional and then longitudinal studies. Higher vital capacity (VC) and forced expiratory volume in 1 s (FEV1; both P < 0.001), and lower residual volume (P < 0.01) were observed in both PFT. Longitudinal analysis showed a smaller VC reduction than FEV1 reduction, leading to a FEV1/VC percentage decrease with time. Maximal mid expiratory flow (MMEF) and MMEF/VC changes during this 9-year period showed an unusually pronounced decrease, suggesting possible chronic effects of diving on small airways. Thus, it is suggested from our observations that a hyperbaric stimulus compensates in part for the effects of aging on VC and that obstructive disease could occur in subjects with long diving experience.