The dawn phenomenon is a condition recently described in patients with insulin-dependent diabetes mellitus (IDDM) that is characterized by abrupt increases in fasting levels of plasma glucose or insulin requirements or both between 5 and 9 a.m., in the absence of antecedent hypoglycemia. To determine its potential clinical relevance, we assessed its frequency and reproducibility in 20 patients with IDDM and in 13 patients with non-insulin-dependent diabetes mellitus (NIDDM) during overnight closed-loop (feedback-controlled) intravenous insulin infusion. After 6 a.m., plasma glucose levels increased similarly in NIDDM (89 +/- 2 mg per deciliter, midnight to 6 a.m., vs. 98 +/- 2 mg per deciliter, 6 to 9 a.m.; P less than 0.01). Insulin requirements increased at least 50 per cent for 1 1/2 hours in 77 per cent of patients with NIDDM and in 75 per cent of patients with IDDM. In five patients with IDDM who were studied on four occasions, the phenomenon occurred during 17 of the 20 observation periods, with insulin requirements after 6 a.m. increasing 225 +/- 34 per cent; coefficients of variation in individual patients ranged from 4 to 25 per cent. Thus, the dawn phenomenon occurs commonly in both NIDDM and IDDM, but its potential variability must be taken into consideration when one is attempting to adjust insulin doses.