Troglitazone was recently reported to specifically promote the differentiation of pre-adipocytes into adipocytes in vitro in subcutaneous fat only, indicating a relation to insulin-resistance-improving action of troglitazone. To expand on this finding, we investigated at the clinical level how long-term administration of troglitazone influences the body fat distribution in type 2 diabetic patients. Troglitazone (400 mg/day) was administered for 6 months to 30 type 2 diabetic patients whose glycemic control was poor. A total of 18 patients received diet therapy alone (in the single-treatment group, BMI 26.0 +/- 4.6, HbA1c 8.2 +/- 1.7%), and 12 patients concomitantly received glibenclamide (1.25-7.5 mg/day) (in the concomitant sulfonylurea group, BMI 25.4 +/- 4.7, HbA1c 9.2 +/- 1.2%). BMI, HbA1c, serum lipid level, and body fat distribution, which were determined by computed tomography (CT) scan at the umbilical level, were measured and compared before and after troglitazone treatment. During the 6-month troglitazone treatment, HbA1c levels decreased and BMI increased in both groups. As for body fat distribution in the single-treatment group, visceral fat area (VFA) decreased (from 118.3 +/- 54.3 to 101.1 +/- 50.8 cm2; P < 0.001), and subcutaneous fat area (SFA) increased (from 189.7 +/- 93.3 to 221.6 +/- 101.6 cm2; P < 0.001), resulting in a decrease in visceral/subcutaneous (V/S) ratio (from 0.74 +/- 0.48 to 0.50 +/- 0.32; P < 0.001). In the concomitant sulfonylurea group, VFA was unchanged (from 108.1 +/- 53.5 to 112.5 +/- 59.9 cm2), while SFA increased (from 144.6 +/- 122.0 to 180.5 +/- 143.5 cm2; P < 0.01), thereby decreasing the V/S ratio (from 0.91 +/- 0.46 to 0.77 +/- 0.44; P < 0.01). The serum triglyceride level and the area under glucose curve during the 75-g oral glucose tolerance test decreased significantly in the single-treatment group. According to our data, troglitazone appears to promote fat accumulation in the subcutaneous adipose tissue rather than in the visceral adipose tissue in mildly obese Japanese people with type 2 diabetes. This shift of energy accumulation from the visceral to subcutaneous adipose tissue may greatly contribute to the troglitazone-mediated amelioration of insulin resistance.