1. Blind analysis of contrast echocardiograms to detect intracardiac shunts, blind analysis of lung function tests for evidence of small airways disease, smoking history and dive characteristics were examined in an attempt to explain neurological symptoms that occurred within 5 min of surfacing from unprovocative dives. 2. Pulmonary abnormalities were significantly more frequent in those divers without intracardiac shunts (50%) than in those with shunts (0%). Smoking was more common in those divers without shunts (55% versus 15%), although this just failed to reach conventional significance levels. Divers without shunts experienced cerebral rather than spinal symptoms after significantly shallower dives with lower tissue nitrogen loads. Depths of dives, tissue nitrogen loads and clinical manifestations in those divers without shunts were similar to the findings in divers who had symptoms after rapid ascents. Despite conservative dive profiles, clinical manifestations in divers with shunts resembled those observed after missed decompression stops. 3. The findings suggest that occult lung disease, and probably smoking, increase the risk of neurological symptoms, even after unprovocative dives, and the similarity of the dive profiles and clinical manifestations to cases with rapid ascents suggest that pulmonary barotrauma and arterial gas embolism are responsible. In divers with intracardiac shunts the different dive profiles and clinical manifestations imply that there is another mechanism, involving different tissue and bubble nitrogen kinetics resulting in venous gas liberation and peripheral amplification in embolized tissues, rather than paradoxical embolism per se.