Because weight loss is difficult to achieve, it would be helpful to determine whether there are subgroups of obese type II diabetic patients who benefit most from participation in a behavioral weight loss program. We studied 178 obese patients with type II diabetes, who participated in a 12-20 week behavioral weight loss program and were followed for 1 year after the program to determine whether age, gender, percent overweight, medication, duration of diabetes or fasting glucose were related to weight loss and/or to the magnitude of improvement in glycemic control experienced with weight loss. Gender was the only variable related to weight loss; males lost more weight and had greater decreases in percent overweight than females. The variable most strongly related to improvement in glycemic control was pretreatment fasting glucose level; patients with higher initial glucose levels experienced the greatest improvements in control. There was no evidence to support the belief that patients on insulin have poorer weight losses or that patients with long-duration diabetes benefit less from weight reduction than those with recent-onset diabetes.