AIMS: To evaluate, in patients with type 2 diabetes (T2DM) treated with insulin, the extent of weight gain over 2 years of insulin treatment, and the dynamics of weight gain in relation to glycaemic achievements over time according to adiposity levels at insulin initiation. MATERIALS AND METHODS: Patients with T2DM (n = 155 917), who commenced insulin therapy and continued it for at least 6 months, were selected from a large database of electronic medical records in the USA. Longitudinal changes in body weight and glycated haemoglobin (HbA1c) according to body mass index (BMI) category were estimated. RESULTS: Patients had a mean age of 59 years, a mean HbA1c level of 9.5%, and a mean BMI of 35 kg/m2 at insulin initiation. The HbA1c levels at insulin initiation were significantly lower (9.2-9.4%) in the obese patients than in patients with normal body weight (10.0%); however, the proportions of patients with HbA1c >7.5% or >8.0% were similar across the BMI categories. The adjusted weight gain fell progressively with increasing baseline BMI category over 6, 12 and 24 months (p < .01). The adjusted changes in HbA1c were similar across BMI categories. A 1% decrease in HbA1c was associated with progressively less weight gain as pretreatment BMI rose, ranging from a 1.24 kg gain in those with a BMI <25 kg/m2 to a 0.32 kg loss in those with a BMI > 40 kg/m2 . CONCLUSIONS: During 24 months of insulin treatment, obese patients gained significantly less body weight than normal-weight and overweight patients, while achieving clinically similar glycaemic benefits. These data provide reassurance with regard to the use of insulin in obese patients. © 2016 John Wiley & Sons Ltd.