ABSTRACT Exchangeable potassium (Ke) and total body water (TBW) were determined before treatment and 5–46 months after therapeutically induced remission in 12 cases of Cushing's syndrome. In six patients, Ke was determined repeatedly after treatment. From these measurements changes in body cell mass (BCM), body fat and in the volume of extracellular water (ECW) were estimated. BCM increased in all cases but two and this increase was most marked in the patients who showed muscular wasting. This increase often developed slowly and was probably related to physical activity. In six patients suffering from severe Cushing's syndrome, the increase in BCM varied between 1.7–6.9 kg (mean 3.5 kg). Body fat decreased on an average by 6.1 kg. Before treatment, the patients looked obese but the overweight was moderate in relation to height. The calculated ECW did not change significantly in most cases. Only in one case was considerable overhydration present in the absence of heart disease. Significant intracellular deficit of potassium was probably not present in this series of patients.