Self-referred subjects (N = 227) thought to be at increased risk of developing diabetes who had fasting plasma glucose (FPG) values in the range of 5.5 to 7.7 mmol . L - 1 on two consecutive occasions 2 weeks apart were randomized to sulfonylurea therapy (gliclazide, =< 160 mg . d - 1 ) or to a control group allocated either to double-blind placebo or to no tablets. Subjects were randomly allocated also to reinforced or basic healthy-living advice in a factorial design. A total of 201 subjects have been evaluated for 1 year in three English and two French hospital outpatient centers. Those allocated to sulfonylurea had a significant (P < .001) reduction in median FPG compared with the control group (6.0 mmol . L - 1 to 5.6 mmol . L - 1 , P < .001, v 6.0 mmol . L - 1 to 6.0 mmol . L - 1 , NS). Median hemoglobin A 1 c (HbA 1 c ) also improved P < .0002; 5.8% to 5.6%, P < .001, v 5.7% to 5.6%, NS), as did mean β-cell function (62% to 70%, P < .01, v 62% to 61%, NS). Mean body weight was unchanged in subjects allocated to sulfonylurea (81.7 kg to 82.4 kg, NS), but decreased in the control group (81.6 kg to 80.4 kg, P < .01). More subjects in the sulfonylurea group versus the control group reported one or more minor symptoms of hypoglycemia over 1 year (50% v 24%, P < .0001). Only two subjects reported major hypoglycemic episodes requiring assistance, both of whom were taking sulfonylurea. Insulin sensitivity did not change between groups. Sulfonylurea therapy with gliclazide improved glycemic control and β-cell function significantly in subjects with increased but not diabetic FPG levels. The study is being extended to determine whether sulfonylurea therapy prevents progression to non-insulin-dependent diabetes mellitus (NIDDM).