Measurement of heart rate variation during deep breathing has been used as a diagnostic test of autonomic function. Normal subjects have considerable heart rate variation that is accentuated during deep breathing; this variation is diminished or sometimes absent in diabetics with autonomic neuropathy. Recording heart rate variation during deep breathing is a good method for establishing the presence of autonomic neuropathy: All normal subjects have a score greater than 9, and autonomic neuropathy is probably absent if the score is greater than 12. The optimal breathing rate for this test is six breaths per minute in nondiabetics but is usually less in diabetics with autonomic neuropathy. The use of six breaths per minute as a standard, however, enhances the ability of the test to discriminate normal from neuropathic patients. Imparied heart rate variation can be the earliest sign of diabetic neuropathy and may precede the appearance of autonomic symptoms by several years. Severe autonomic neuropathy may be responsible for spontaneous respiratory arrest and unexplained sudden deaths, which are not rare among these patients.