OBJECTIVE - To examine day-night blood pressure (BP) variation in normotensive, normoalbuminuric subjects with type I diabetes and to assess the prevalence of an impaired nocturnal BP fall. RESEARCH DESIGN AND METHODS - Fourteen healthy volunteers and 13 normotensive, normoalbuminuric subjects with type I diabetes were studied with an ambulatory sphygmomanometric device for 24 h. RESULTS - No significant difference was found between diabetic and control groups with regard to daytime systolic blood pressure (sBP) or diastolic blood pressure (dBP). However, the mean nighttime sBPs (P < 0.01) and dBPs (P = 0.01) were significantly higher in the diabetic group compared with the control group. Furthermore, the night/day ratio for both sBP (P < 0.01) and dBP (P < 0.01) was significantly higher in the diabetic group. Approximately half of the diabetic subjects studied were non-dippers when defined either by a nocturnal fall in sBP/dBP of <10/5 mmHg (5/13 vs. 0/14, P < 0.05: diabetic group vs. control group) or a day to night fall in either sBP (7/13 vs. 2/14, P < 0.05) or dBP (6/13 vs. 0/14, P < 0.01) of <10%. CONCLUSIONS - Ambulatory BP may reveal impaired nocturnal BP fall in normotensive, normoalbuminuric subjects with type I diabetes. These subjects may be at greater risk of certain complications as a consequence of an increased BP burder.