This review presents recent data on weight gain when on insulin treatment in type 1 and type 2 diabetic patients. In both types of diabetes, the excess weight gain with intensified insulin therapy compared with conventional insulin or sulfonylurea remains modest: 2.6 kg over 7.5 years in the Stockholm study (type 1 diabetic patients) and 1.7 kg when compared to glibenclamide over 10 years in the UKPDS (type 2 diabetic patients). Patients who gain the most weight after insulin initiation are those who: (1) had the worst metabolic control before the intensification of treatment, (2) had the greater weight loss prior to insulin initiation, and (3) in the case of patients with type 1 diabetes, have a family history of type 2 diabetes. This suggests that most of the weight gain observed after the insulin initiation is a "catch-up" weight re-gain. There is no evidence that weight gain after insulin therapy initiation is associated with a deterioration in the lipid profile or arterial hypertension or an excess risk for cardiovascular events, contrary to common beliefs. All clinical studies performed to date with the insulin analogue detemir have shown that this analogue is associated with lesser weight gain than NPH insulin. There is no explanation yet for these intriguing results. If confirmed on the long-term, this favourable effect on weight might be an interesting feature of this new insulin analogue.