To determine whether diabetic individuals have more difficulty losing weight than nondiabetic individuals, 12 overweight diabetic subjects (6 men, 6 women) and their overweight nondiabetic spouses were treated together in a behavioral weight-control program. Diabetic and nondiabetic subjects did not differ in age, weight, or percent overweight. Weight losses of nondiabetic spouses were significantly greater than those of diabetic patients (13.4 +/- 1.7 vs. 7.5 +/- 1.4 kg; P less than .01). Differences emerged by wk 5 and became greater over the 20-wk program. Nondiabetic subjects reduced their intake significantly more than diabetics, suggesting that differences in dietary adherence were responsible for the differences in weight loss.