The effect of a low-sucrose, low-fat diet on insulin sensitivity, insulin binding to monocytes, and insulin secretion in nonketotic diabetic patients was studied. Ten obese diabetics were studied for 1 yr before and during treatment with a 1200-1500-kcal diet, whereas six diabetics of normal weight were studied for 3 mo before and after treatment with a 2200-2400-kcal diet. In the obese group, no change was found in the insulin response to i.v. injection of glucose during treatment (p greater than 0.1), but the insulin sensitivity was normalized after 1 yr (p less than 0.01). The clinical normalization and the improvement of insulin sensitivity were accompanied by a parallel normalization of the binding of insulin to monocytes (p less than 0.01). In the group of normal-weight diabetics, both the insulin sensitivity (p less than 0.05) and the insulin binding to monocytes (p less than 0.05) were normalized after a 3-mo treatment period, but the insulin secretion increased (p less than 0.05) without reaching normal values. We conclude that most nonketotic diabetic patients can be controlled by diet treatment alone. The mechanism of action of the low-fat, low-sucrose diet seems for the greatest part to be a normalization of the insulin sensitivity, which is partly caused by a normalization of the cellular insulin binding.