Insulin treatment is well known to induce progressive body weight gain. However, rapid weight increase due to transient fluid accumulation is rare. Bioelectrical impedance analysis (BIA) is a convenient method for determining body composition and water content. We report an 18-year-old diabetic female with rapid insulin-induced weight gain due to excessive body water retention, found by serial BIA measurement. The patient was admitted to our hospital due to uncontrolled diabetes. She had an initial body weight of 55 kg and height of 165 cm. However, a weight gain of 6.5 kg was noted one week after starting insulin injections and further increased to 8 kg after the second week. Finally a net weight increase of 4 kg from fat and lean mass was attained after two months. The weekly BIA data showed that most of the initial weight gain came from water retention, peaking on day 14 and recovering afterwards. Rapid weight gain shortly after insulin therapy may be due to excessive but reversible water retention, detected by repeated BIA measurements.