Immersion induces air trapping in the lungs, as does asthma. Consequently, when using diving apparatus, asthmatics may face greater risk than non-asthmatics of pulmonary barotrauma (PBT) during ascent. We studied the pulmonary airflows and closing capacities (CC = closing volume + residual volume) in subjects with exercise-induced asthma (A, n = 12) and in healthy controls (C, n = 11) under four conditions: dry and immersed, both before and after exercise (treadmill running, non-immersed). Immersed, both C and A had significant and equivalent reductions in vital capacity, FEV1, FEV1/FVC, and FEF25%-75%. Post-exercise and immersed, pulmonary airflows deteriorated further in A but were better in C: FEV1 (A, 3.6 +/- 0.8 liter vs. 3.3 +/- 0.8 liter, P = 0.001; C, 3.9 +/- 0.5 liter vs. 4.1 +/- 0.6 liter, P = 0.006), FEF25-75% (A, 3.5 +/- 1.0 liter x s(-1) vs. 3.0 +/- 0.8 liter x s(-1). P < 0.05; C, 4.0 +/- 0.9 liter x s(-1) vs. 4.3 +/- 0.9 liter x s(-1), P < 0.05). Therefore, in contrast to C, A subjects had reduced pulmonary airflows during immersion after exercise. Furthermore, A subjects more often had no closing volume phase IV when immersed after exercise than C (P = 0.005). Interpreting the absence of phase IV as indicative of more air trapping in the asthmatics during immersion after exercise would be consistent with the reductions in airflow.