Nineteen diabetic patients, 12 type I (insulin-dependent) and 7 type II (late-onset, non-insulin-dependent), underwent nocturnal polygraphic monitoring after a daytime medical evaluation that included tests of vagal responses and, in 6 patients, pulmonary function and hypercapnic and hypoxic responses. Five lean type I patients had abnormal sleep-related breathing patterns with central or obstructive sleep apnea and brief breathing irregularities during stages 3 to 4 non-rapid-eye-movement sleep, compared with only 1 overweight type II diabetic with moderate obstructive sleep apnea. There was no correlation between daytime ventilatory study findings and abnormal breathing patterns during sleep, but there was a clear relationship between neuropathy and sleep-related breathing abnormalities in type I insulin-dependent diabetics.