To clarify the issue of pulmonary dysfunction in diabetes mellitus, lung mechanics and CO transfer were investigated in 22 young (mean age 19.5 +/- 5 years) non-smoking, insulin-dependent diabetic patients and an equal number of matched healthy subjects. Mean closing capacity/total lung capacity (CC/TLC) was significantly greater in the diabetic than in the control group (31.4 +/- 6.8 vs 27.2 +/- 2.9 percent, p less than 0.01), as was the mean value of the volume independent index of lung elasticity (exponent constant, Kst(L)) (0.148 +/- 0.045 vs 0.118 +/- 0.030, p less than 0.05). The transfer factor expressed per unit alveolar volume (TL/VA) was also significantly lower in the diabetic than in the control group (5.25 +/- 0.68 vs 5.61 +/- 0.57 ml/min/mm Hg/L, p less than 0.05) and this could be ascribed to a lower pulmonary capillary blood volume. There was evidence of mildly abnormal lung mechanics and/or a decreased pulmonary capillary blood volume in 16 (73 percent) of the diabetic group. Since pulmonary dysfunction was either an isolated non-endocrine finding or was associated with only early systemic complications in these young patients, our findings suggest that pulmonary dysfunction is an early measurable complication in insulin-dependent diabetes mellitus.