This study compared the effect of increased CO2 partial pressure in atmosphere up to 30 mm Hg (i.e. hypercapnia) on the risk of decompression sickness (DCS) during decompressions simulating space extravehicular activity (EVA). Test subjects were exposed to a pressure 330, 308 and 261 mm Hg, breathed O2-CO2 gas mixture under experimental or 100% O2 control conditions. DCS symptoms (joint pains) developed in 32.7% (33/101) of control and 67.9% (72/106) of experimental exposures (p<0.05). This finding is of importance for delivering medical support during EVA.