To investigate one suggested cause of unexplained deaths of diabetic patients with autonomic neuropathy, ventilatory responses to progressive hypoxemia and to progressive hypercarbia were compared among two groups of diabetic patients, with and without autonomic neuropathy, and a group of normal control subjects. Hypoxemia was induced gradually under isocapnic conditions and the arterial oxygen saturation was reduced to below 75%. In a separate test the end tidal CO2 was increased gradually to 55 mm Hg in subjects who could tolerate this degree of hypercarbia. The ventilatory responses to hypoxemia and to hypercarbia did not differ among groups nor did age, duration of diabetes, or presence of proliferative retinopathy and nephropathy have a significant effect on the ventilatory responses of diabetics. The authors conclude that defective ventilatory responses to hypoxemia or hypercarbia are not associated with the sudden unexplained deaths in diabetics with autonomic neuropathy.