To assess the effectiveness of the Pritikin program of diet and exercise for treating patients with non-insulin-dependent diabetes mellitus (NIDDM), data were obtained from 60 patients who completed the 26-day residential program. Of the 23 patients who were taking oral hypoglycemic agents upon entry, all but 2 were off medication by the end of the program. Of the 17 patients who were taking insulin, all but 4 were off medication at discharge. Two of the four had their insulin reduced by 50% while the remaining two had no major change in their insulin dosage. Fasting blood glucose was reduced from 194.9 +/- 10.1 to 144.6 +/- 7.1 mg/dl. Serum cholesterol was reduced from 225.4 +/- 5.7 to 181.7 +/- 4.9 mg/dl while triglycerides were reduced from 283.7 +/- 28.8 to 186.2 +/- 11.6 mg/dl. The group as a whole lost an average of 4.3 kg/body wt and achieved 40.5% of their desired weight loss. Maximum work capacity increased from 5.6 +/- 0.3 to 7.9 +/- 0.4 METs, while daily walking increased from 11.7 +/- 2.4 to 102.8 +/- 4.8 min/day. The decrease in fasting glucose was not correlated with weight loss (r = 0.24), increase in walking time (r = 0.00), or increase in MET capacity (r = 0.05). We conclude that the total program is an effective means for treating NIDDM patients. We also feel that the high-complex-carbohydrate, high-fiber, low-fat diet is of primary importance.