Thirty-eight consenting subjects scheduled for elective ophthalmologic surgery were classified as nondiabetics (n = 21) or diabetics (n = 17) and were tested preoperatively for autonomic dysfunction. The autonomic tests consisted of respiratory sinus arrhythmia and heart rate responses to the Valsalva maneuver to test cardiac vagal function and diastolic blood pressure responses to head-up tilt and cold pressor test to assess sympathetic efferent integrity. At a separate time, anesthesia was established with fentanyl (2 micrograms/kg), sodium thiopental (3-5 mg/kg), and vecuronium (0.1 mg/kg), and maintained with isoflurane, oxygen, and nitrous oxide. An anesthesiologist, blinded to the autonomic test results, recorded perioperative blood pressure and heart rate. The autonomic test results revealed significant autonomic dysfunction among the diabetics. Heart rate and blood pressure declined to a greater degree (P less than 0.05) during induction of anesthesia in diabetics compared with controls and there was less of an increase in these same parameters following tracheal intubation in diabetic patients. Thirty-five percent of diabetics required intraoperative vasopressors compared with only 5% of control patients (P less than 0.05). A major finding was that the diabetics who required intraoperative blood pressure support had significantly greater impairment of autonomic test results compared with those diabetics who did not need vasopressors. Diabetics are at increased risk for cardiovascular lability during anesthesia and preoperative screening of diabetics with simple noninvasive autonomic tests may be useful in identifying those at high risk for perioperative cardiovascular instability.