In order to investigate pulmonary gas exchange and cardiopulmonary performance in Type 1 diabetes, 36 patients underwent a progressive incremental exercise test on a cycle ergometer. Cardiopulmonary variables were measured, and arterial blood gases determined on samples obtained from an indwelling catheter in the radial artery. The results were compared with those from 40 control subjects. In the patients, the maximum power (Wmax) and maximum oxygen uptake (VO2max) were lower than in the control subjects (Wmax 186 +/- 52 (+/- SD) vs 233 +/- 48 W, p less than 0.05; VO2max 2.56 +/- 0.71 vs 3.17 + 0.77 l min-1, p less than 0.05). At comparable levels of power output, however, no significant abnormality was observed in the difference between alveolar and arterial oxygen pressure (P(A-a)O2), and the ratio of physiological dead space to tidal volume (VD/VT ratio). These data indicate that in Type 1 diabetic patients, despite their reduced maximum oxygen uptake, gas transfer during exercise is not limited and thus does not contribute to the impairment of exercise capacity.