Sixteen episodes of severe hypercalcaemia (more than 3.25 mmol/l) were treated by rehydration alone. Sodium repletion was invariably achieved within 48 hours (mean deficit 9.24 mmol/kg) although the fall in serum calcium was more protracted. A substantial fall in serum calcium (mean decrease 0.6 mmol/l) was achieved in thirteen patients; poor responses in three patients were associated with a rapidly increasing calcium load. Presentation of the data in terms of calcium excretion per unit of glomerular filtrate (CaE) and the setting of tubular reabsorption (TmCa/GFR) makes it possible to predict the likely effects of rehydration and patients with non-metastatic hypercalcaemia are easily identified. Rehydration is simple and often effective in the early management of this common metabolic problem but it is important that therapeutic goals are realistic and the intrinsic limitations of rehydration recognized. This depends upon a clear idea of the contribution that the kidney makes to the hypercalcaemia of malignant disease.