We measured serially the blood pressure of 7 normal controls and 11 Type I insulin-dependent and 6 Type II non-insulin-dependent diabetic patients during sleep. All subjects underwent 2 or 3 nights of monitoring during which blood pressure was measured with an arteriosonde, a Doppler-type system with a cuff that automatically inflates every 15 minutes. All 7 normal controls, 3 of 11 Type I, and all 6 Type II diabetic patients had normal results from bedside tests of autonomic function. The Type I patients had abnormal systolic blood pressure measurements throughout the night. Unlike normal controls and Type II diabetics, their blood pressure did not rise progressively in the early morning hours. During rapid eye movement (REM) sleep, their blood pressure measurements were significantly higher and lower than values measured during non-REM sleep, in sharp contrast to those in the other two groups, whose REM sleep-related blood pressure values were slightly above the preceding non-REM sleep measurements.