The prevalence of pulmonary oedema during scuba-diving is unknown. In our referral centre for diving accidents we have observed several episodes of pulmonary oedema in four previously healthy persons while scuba-diving or swimming. Four events were documented by physical findings, typical chest radiographic changes, and arterial hypoxaemia. Four additional episodes were identified in one of the individuals by a suggestive history. No technical problems with the diving equipment were detectable and none of the individuals reported aspiration of water. In order to gather information about the incidence of pulmonary oedema, we carried out a survey among 1,250 divers. To elucidate possible underlying mechanisms of this complication we investigated forearm vascular resistance, levels of vasoreactive hormones, and left ventricular function by Doppler echocardiography, at room temperature and during cold exposure, in four patients and in healthy control subjects. We found only one additional person with a history suggestive of pulmonary oedema among 460 responders to the survey. We found no differences in forearm vascular resistance, left ventricular systolic and diastolic function, and plasma levels of epinephrine, norepinephrine, cortisol, aldosterone, renin and atrial natriuretic peptide between the patients with a history of pulmonary oedema and the control subjects. We conclude that the occurrence of pulmonary oedema during scuba-diving or swimming is an extremely rare event in healthy individuals. The mechanisms responsible remain unclear.