Deep sea diving might cause tremendous physical or psychological stress to divers. The present study aims to evaluate the pulmonary stress experienced by navy divers after a simulated deep wet dive. Nineteen navy divers took part in this study during their annual deep dive training. Ten divers were exposed to 190 feet of sea water (fsw) breathing compressed air on day 1 and to 250 fsw breathing helium-oxygen (Heliox) gas mixture on day 3. Other nine divers were exposed to 220 fsw on day 1 and 285 fsw on day 3 breathing Heliox gas mixture. The bottom time ranged from 5 to 8 min and the standard U.S. Navy Air and Surface-Supplied Helium Oxygen Decompression Tables were then followed for surfacing. Air dive to 190 fsw caused a signifi cant decrease in the FEV1/FVC, but no statistically signifi cant difference in the FEV1. On the other hand, the FEV1 increased signifi cantly after a Heliox dive to 220 fsw, but not after dives to 250 or 285 fsw. Our results suggested that deep diving using different gas mixtures may have diverse impacts on pulmonary functions. Therefore, strict criteria on pulmonary function test may be necessary in the fi tness recommendation for recruitment of deep divers.