Type 2 diabetes mellitus is increasing worldwide largely as a result from increasing obesity and sedentary lifestyle. The Finnish Diabetes Prevention Study (DPS) is the first individually randomized controlled clinical trial to test the feasibility and efficacy of lifestyle modification in high-risk subjects. We randomly assigned 522 (172 men, 350 women) middle-aged (mean age 55 yr), overweight (mean body mass index 31 kg/m(2)) subjects with impaired glucose tolerance either to the lifestyle intervention or control group. Each subject in the intervention group received individualized counseling aimed at reducing weight and intake of total and saturated fat, and increasing intake of fiber and physical activity. An oral glucose tolerance test was performed annually to detect incident cases of diabetes and to measure changes in metabolic parameters. The mean (+/- SD) weight reduction from baseline to year 1 and to year 2, respectively, was 4.2 +/- 5.1 kg and 3.5 +/- 5.5 in the intervention group and 0.8 +/- 3.7 kg and 0.8 +/- 4.4 in the control group (P < 0.001 between the groups). At the time of first analysis of the outcome data the mean duration of follow-up was 3.2 yr. The risk of diabetes was reduced by 58% (P < 0.001) in the intervention group compared with the control group. The reduction in the incidence of diabetes was directly associated with number and magnitude of lifestyle changes made. In conclusion, the DPS is the first controlled trial demonstrating that type 2 diabetes can be prevented by changes in lifestyle in high-risk subjects.