Most patients with type 2 diabetes gain weight when treated with insulin. Weight gain is observed when insulin is introduced after oral agents have failed, but also when insulin is introduced shortly after the diagnosis of diabetes. The mechanisms of this weight gain are incompletely understood, but reduction of energy lost by glucosuria and reduction of energy needed for glucose production are main determinants. The same reasons apply to the weight gain observed at the beginning of treatment with sulfonylureas, even though patients usually gain less weight with sulfonyulreas than with insulin. In the UKPDS, at 10 years of the study, patients treated with insulin gained 2 kg more, i.e. 2.5% of the average weight of patients included in the trial, than patients treated with sulfonylureas. The reasons of the excess of weight gain with insulin as compared with sulfonylureas remain unclear. Patients with type 2 diabetes treated with insulin gain weight only during the first 2-3 years after insulin introduction. The weight stabilizes thereafter. Type 2 diabetes usually remains unknown for years before diagnosis and patients may lose weight during this long period of time preceding diagnosis. It is hypothesized that the weight gain observed after the introduction of insulin may simply correspond to the reexpression of the physiologically controlled body weight.