We report a patient with Prader-Willi syndrome (PWS) complicated by diabetes mellitus. PWS is a genetic disorder characterized by obesity, mental retardation and hypogonadism. Glucose intolerance in this syndrome is thought to be secondary to insulin resistance associated with morbid obesity. Therapy was directed primarily at decreasing insulin resistance and thereby improving glucose intolerance by the administration of troglitazone, which increases insulin sensitivity. Changes in glucose disposal rate assessed by euglycemic hyperinsulinemic clamp test were measured, as well as glucose and insulin responses to a 75 g-OGTT before and after troglitazone therapy. Glucose disposal rate increased by 36% and plasma glucose responses to 75 g-OGTT decreased by about 50% during 12 weeks of troglitazone therapy despite slight weight gain. Thus, troglitazone has beneficial effects on glycemic control by improving insulin sensitivity in patients with PWS complicated by diabetes mellitus.