OBJECTIVE: To determine whether diet and endurance exercise improved adiposity-related measurements in Japanese Americans with impaired glucose tolerance (IGT). RESEARCH DESIGN AND METHODS: This study compared the effects of an American Heart Association (AHA) step 2 diet (<30% of total calories as fat, <7% saturated fat, 55% carbohydrate, and < 200 mg cholesterol daily) plus endurance exercise for 1 h three times a week (treatment group) with an AHA step 1 diet (30% of total calories as fat, 10% saturated fat, 50% carbohydrate, and <300 mg cholesterol) plus stretching exercise three times a week (control group) on BMI, body composition (% fat), and body fat distribution at 6 and 24 months of follow-up in 64 Japanese American men and women with IGT, 58 of whom completed the study. RESULTS: At 6 months, the treatment group showed significantly greater reduction in percent, body fat (-1.4 +/- 0.4 vs. -0.3 +/- 0.3%); BMI (-1.1 +/- 0.2 vs. -0.4 +/- 0.1 kg/m(2)); subcutaneous fat by computed tomography at the abdomen (-29.3 +/- 4.2 vs. -5.7 +/- 5.9 cm(2)), thigh (-13.2 +/- 3.6 vs. -3.6 +/- 3.0 cm(2)), and thorax (-19.6 +/- 3.6 vs. -8.9 +/- 2.6 cm(2)); and skinfold thickness at the bicep (-2.0 +/- 0.6 vs. 1.1 +/- 0.6 mm) and tricep (-3.7 +/- 0.8 vs. -0.9 +/- 0.6 mm), which continued despite moving to home-based exercise for the last 18 months. CONCLUSIONS: Diet and endurance exercise improved BMI, body composition, and body fat distribution and, thus, may delay or prevent type 2 diabetes in Japanese Americans with IGT.