Pulmonary function was measured in 152 professional saturation divers and in a matched control group of 106 subjects. Static lung volumes, dynamic lung volumes and flows, transfer factor for carbon monoxide (Tl,), transfer volume per unit alveolar volume (K>), delta-N2, and closing volume (CV) were measured and compared with reference values from recent Scandinavian studies, British submariners, and the European Community for Coal and Steel (ECCS) recommended reference values. Diving exposure was assessed as years of diving experience, total number of days in saturation and depth, and as the product of days in saturation and mean depth. Divers had significantly lower values for forced expired volume in one second (FEVy), FEV,/forced vital capacity (FVC) ratio, FEF25-75% FEF7s45% FEF,(%q FEF7S%, Tly,,, and K,, compared with the controls and a significantly higher CV. There was a positive correlation between diving exposure and CV, whereas the other variables had negative correlations with diving exposure. Values for the control group were not different from the predictive values of Scandinavian reference studies or British submariners, although the ECCS standard predicted significantly lower values for the lung function variables both in divers and the control group. The pattern of the differences in lung function variables between the divers and controls is consistent with small airways dysfunction and with the transient changes in lung function found immediately after a single saturation dive. The association between reduced pulmonary function and previous diving exposure further indicates the presence of cumulative long term effects of diving on pulmonary function.Previous studies have shown that divers in general have larger than predicted vital capacities compared with a standard reference population.'-3 Possible development of bronchial obstruction in divers has been questioned in these studies because the forced expired volume in one second (FEV,)/forced vital capacity (FVC) ratio was lower than predicted, and in one study the forced expiratory flow rate (FEF) at 75% of FVC expired (FEF75%) was also shown to be lower than predicted.' This pattern of changes may result from the larger than predicted vital capacity, as there is a negative correlation between FVC and FEV1/FVC ratio,4 and thereby a result of respiratory muscle training because of breathing dense gases, or simply a result of the selection process of divers. There is a thorough screening of divers for cardiopulmonary disorders both at the start of their education and by annual medical examinations in both Norway and Britain. Effects of a single deep dive on pulmonary function, however, have been shown in several studies.'' A reduction in transfer factor for carbon monoxide (Tl,O) is the most consistent effect reported. A small increase in vital capacity is often, but not always seen, and in two studies an increase in residual volume and total lung capacity was found.67 Gas exchange abnormalities, changes in ventilatory requirements and physiologi...