Aim. To investigate diffuse capacity and static pulmonary volumes in type I diabetics. Material and methods. We investigated 141 young (66 females) patients with long-standing type I diabetes mellitus (IDDM) (mean age 26 2, 6 2 years, mean duration of endocrinopathy 12.6, 7.3 years) in comparison to 36 age- ,sex-, weight-and height-matched healthy controls (Q. The diabetic patients were divided into 4 groups: 1) 46 diabetic patients without microvascular complications (DPI), 2) 48 diabetic patients with debut late complication (DP2), 3) 34 diabetic patients with severe late complications (DP3), 4) 13 diabetic patients with end-stage renal failure (nephropathy stage 5) who were not yet on dialysis (DP4). Spirometry measurements before and after bronchodilator administration were performed, TLco was measured in sitting position by the single-breath (TLcosb) and steady-state (TLcoss) methods, TLcosb corrected by alveolar volume (KCO = TLcosb/VA), single breath helium dilution lung volumes were done. Results. There was no difference in gas exchange and lung volumes between DPI and C. We found a significant reduction of TLcosb, TLcoss, KCO parameters as compared with controls in diabetic patients with initial late complications (DP2). Severe diabetic microangiopath (DP3, DP4) associated with significantly decreased TLco and reduction total lung capacity (TLC). Typical features of restrictive pulmonary defects, namely a reduction of TLC and TLco were observed predominantly in patients with diabetic nephropathy of stage IV-V There was no evidence of air flow limitation in any of diabetics studied. Conclusion. Diabetic process caused decreased pulmonary gas exchange and reduction of lung volumes. These gasometric and spirometric changes indicated some coexistence with late diabetic complications.