The study examined pulmonary function parameters of 36 patients with insulin-dependent diabetes mellitus and analyzed their inspiratory muscle performance. The control group consisted of 40 healthy reference persons of a sex ratio, age, height, and weight distribution similar to those of the patients. The pulmonary function test included the measurement of the lung volumes and the maximal expiratory flow-volume curves. The values of maximal sniff esophageal (Pes) and transdiaphragmatic pressures (Pdi) were used as parameters for global inspiratory muscle strength and diaphragm strength, respectively. The 12-s maximum voluntary ventilation (MVV) test supplied the parameter of inspiratory muscle endurance. The diabetic patients showed a highly significant decreased value for their inspiratory vital capacity (VCin) compared with that of the control subjects (4.75 +/- 0.84 versus 5.36 +/- 1.37 L; p less than 0.01). Inspiratory muscle performance in the diabetic patients was also restricted. Sniff Pes was significantly lower in the diabetic group (p less than 0.05); sniff Pdi (p less than 0.01) and MVV (p less than 0.05) were also low. The results did not correlate with either the duration of diabetes or the quality of metabolic control measured by glycosylated hemoglobin concentration. The reduction of VCin in diabetic patients may have been caused partly by the reduced capacity of the inspiratory muscles.