The aim of this study is to clarify the relationship between the efficacy of sulfonylureas and duration of diabetes in elderly diabetics. Daily blood glucose profiles were measured in 87 Type 2 elderly diabetic patients on sulfonylureas (tolbutamide, gliclazide or glibenclamide). Plasma glucose concentrations were determined at 08.00 (before breakfast), 10.00, 12.00 (before lunch), 14.00, 18.00 (before dinner), 20.00, 24.00, 03.00, 06.00, 08.00 hours. The subjects were divided into 4 sub-groups, according to their duration of the diabetes (< 10, 10-14, 15-19, 20 or more years). Mean plasma glucose values at 08.00, 10.00, 20.00, 03.00 and 06.00 hours were not significantly different among the four groups. However, mean plasma glucose values at 12.00, 14.00, 18.00, 00.00 hours and mean total blood glucose area under the daily profile (total BG) were significantly different among the four groups and the values in patients with a history of diabetes of 15 years or more increased. Duration of diabetes positively correlated with blood glucose values at 12.00, 14.00, 18.00, 00.00, 03.00 hours and total BG, and the dose of sulfonylureas positively correlated with blood glucose values at 12.00, 14.00, 18.00, 00.00 hours and total BG in multiple regression analysis. These results suggest that duration of diabetes and dose of sulfonylureas are important determinants of blood glucose control with sulfonylureas in elderly diabetic patients.