Obesity is often associated with type 2 (non insulin-dependent) diabetes. A growing body of evidence support the hypothesis that these two diseases share a common pathogenesis. Nevertheless, experience derived from clinical observation on type 2 diabetic patients indicates that reduction of body weight is not always accompanied by an improvement in metabolic control and that a good metabolic control is often obtained without influencing body composition. Aim of the present study was to evaluate the relationship between body mass and glycemic control in a type 2 diabetic population by a 3 years observational study. A cohort of 562 subjects was studied. At entry more than 80% of patients were overweight or obese according to the body mass index (BMI) scale and this proportion was not significantly reduced at the end of the follow-up. At entry all patients had a glycosylated hemoglobin (HbA1c) value above 8.1% whereas at the end of follow-up more than 2/3 of patients were in good metabolic control. No relationship was observed between modification of body mass and metabolic control. These data confirm the high frequency of obesity among type 2 diabetic individuals but they suggest that impaired glucose metabolism and alteration of body weight have different pathogenesis.