To investigate the effectiveness of an intensive diet and exercise program for controlling non-insulin-dependent diabetes mellitus (NIDDM) and reducing risk factors associated with macrovascular complications. Medical charts obtained from 4,587 participants in a lifestyle modification program were screened for patients with NIDDM. A total of 652 patients was identified, and their responses to the 3-week program were analyzed. Fasting glucose level was reduced from 10.0 to 8.45 mmol/l, and 71% of 197 subjects taking oral hypoglycemic agents and 39% of 212 taking insulin were able to discontinue their medication. Of the 243 not taking medication, 76% reduced their fasting glucose levels to < or = 7.84 mmol/l. Blood pressure was significantly reduced, and of the 319 initially taking antihypertension drugs, 34% had their medication discontinued. Serum total and low-density lipoprotein cholesterol were reduced by 22% and triglycerides by 33%. The ratio of total to high-density lipoprotein cholesterol was reduced by 13%. Lifestyle modification consisting of diet combined with aerobic exercise can be effective for controlling NIDDM and reducing risk factors associated with macrovascular complications in both men and women. The program was far more effective in controlling the disease in patients taking no medication or oral agents compared with patients taking insulin. These results stress the need for early emphasis on lifestyle modification in the treatment of NIDDM.