We studied the effects of prone immersion on pulmonary function in children with asthma. Twelve children with asthma were subjected to prone immersion (WET) and standing upright on land (DRY) under controlled conditions regarding temperature and relative humidity of the inhaled air, minute ventilation, respiratory frequency, tidal volume, and the concentration of carbon dioxide in the exhaled air. Eight minutes of isocapnic hyperpnea was performed in random order either in the swimming pool (prone) or on land (upright), with the temperature and relative humidity of the inhaled air kept at 20 +/- 2 degrees C and 10 +/- 2%, respectively. The average accumulated ventilation for the 8 min of hyperpnea was 3151 for the WET treatment and 3101 for the DRY (P > 0.1). Average heart rates were 101 +/- 11 and 115 +/- 17 beats.min-1 for the WET and DRY treatments, respectively (P < 0.05). The decrease in FEV1 (delta FEV1) was 26.3 +/- 16.7% after the WET treatment (compared with pretreatment FEV1), and 26.4 +/- 11.3% after the DRY treatment (P > 0.1 between WET and DRY delta FEV1), with similar trend (insignificant differences between treatments), for delta FVC, delta PEF 50%, and delta PEF 25-75%. It is concluded that airway hyperactivity is not alleviated by whole body prone immersion.