To determine whether a group behavior modification approach might be preferable to individual counseling in the nutritional therapy of non-insulin-dependent diabetes mellitus, 40 adults younger than 65 yr of age with diabetes mellitus who were not receiving insulin were randomized to either a program of individualized dietary review and recommendations or a program of group meetings aimed at controlling the signals leading to overeating and noncompliance with a diabetic dietary regimen. Statistically significant (P less than 0.05) decreases in body weight, sum skin-fold thickness, fasting serum glucose, and serum triglycerides but not LDL-C or HDL-C were observed. The individual counseling group had a greater amount of weight loss than the behavior modification group. There were no significant (P greater than 0.05) differences between the two groups with respect to the biochemical outcome variables. Patient characteristics assessed at entry--namely anxiety, internal versus external locus of control and perceived disease severity, and compliance with advice--were significantly associated with weight loss in the behavior modification group while only the latter index was of value in the individual counseling group. Thus, our use of these programs does not identify a clear advantage of either approach in the nutritional therapy of non-insulin-dependent diabetic patients.