The purpose of the study was to document the incidence and recurrence rate of pulmonary oedema induced by strenuous swimming (SIPO), and to study the changes in relevant physiological parameters. Thirty-five young men were repeatedly examined over a 2-month period after a swimming time trial in the open sea. A tentative diagnosis of SIPO was made when the swimmer reported shortness of breath accompanied by cough. Twenty-nine events of SIPO were diagnosed in 21 individuals (60% incidence). Oxygen saturation was significantly reduced in SIPO. Mean forced vital capacity (FVC) and FEV(1) were significantly lower in the severe SIPO group. Also, mean FVC and mid-expiratory flows (FEF(25-75%)) obtained 12 months earlier during screening for the programme were lower in individuals who later had SIPO. The ratios of post-swim FVC and FEV(1) values to the corresponding selection examination values were lower in the severe SIPO group. Thus volumes decreased in the SIPO group, besides being lower at the start. Shortness of breath and coughing following strenuous swimming were related to hypoxaemia and reduction in lung volumes, suggesting pulmonary oedema. SIPO was a common and often recurrent phenomenon. Lower initial lung volumes and flows might predict future susceptibility to SIPO.