Insulin use is common in type 2 diabetes and is frequently accompanied by weight gain, the composition of which is poorly understood. The present study evaluates insulin-induced body composition changes. Body weight and composition of thirty-two type 2 diabetic patients undergoing their first 12 months of insulin therapy were compared with those observed in thirty-two type 2 diabetic patients previously treated on insulin (minimum 1 year). Body composition was determined by simultaneous body water spaces (bioelectrical impedance analysis) and body density measurements. After 6 months, glycosylated Hb (HbA1c) significantly improved in the newly treated group (P<0.0001), but remained stable in those treated previously. HbA1c did not differ between 6 and 12 months in the two groups. Body weight significantly (P=0.04) changed over 12 months in those newly treated only (+2.8 kg), essentially comprising fat-free mass (P=0.044). Fat mass remained unchanged (P=0.85) as did total body water, while extracellular: total body water ratio tended to increase in those newly treated (P=0.059). Weight changes correlated with HbA1c changes (R2 0.134, P=0.002) in the initial 6 months only. Insulin therapy leads to weight gain (2.8 kg), predominantly fat-free mass, over 12 months. After 6 months, newly treated patients continued gaining weight despite an unchanged HbA1c, suggesting the potential anabolic role of insulin in subsequent gains. Therefore, in the initial 6 months, weight gain can be attributed to a 'glucose control-related effect' and further gain appears to be due to a 'non-glucose control-related' effect of insulin treatment.