A retrospective analysis of spirometric data from divers attending for annual medical examination at intervals from three to nine years was carried out to examine the long term effect of diving on lung volumes. Those divers with records over a three or four year interval (group 1, n = 224) showed a mean reduction of forced vital capacity (FVC) of 240 ml; those with records over a five or more years interval (group 2, n = 123) showed a reduction of FVC of 400 ml. These reductions remained significant when expressed as a percentage of predicted normal values. The reduction of FVC between records did not correlate with the diver's age, maximum operating depth, duration of diving career, or weight change but was positively correlated with the initial FVC. The reductions in FVC were similar in smokers and non-smokers. The change in forced expiratory volume in one second (FEV1) followed a similar pattern but was less pronounced than the effect on FVC. The decline in FVC associated with diving occurs from values of FVC that are above the predicted normal; few values below predicted normal were observed. The effect may represent either a gradual return towards the predicted normal or a pathological reduction in lung volume.