In hypoparathyroidism the absence of parathyroid hormone leads to a reduction in the absorption of calcium by renal tubular cells. In spite of treatment with vitamin D, hypercalciuria persists and normocalcaemia can only be maintained by providing the kidney with a large load of calcium. Thiazide diuretics enhance tubular calcium reabsorption and it has been suggested that they can be used as an alternative to vitamin D. Bendrofluazide in a dose of 10 mg daily was given to 9 patients with severe hypoparathyroidism in addition to their usual treatment with calcium and vitamin D. Following the introduction of Bendrofluazide the calculated renal threshold for calcium reabsorption (TmCa/GFR) increased by a mean value of 0.14 mmol/l, and the mean rise in serum calcium was 0.13 mmol/l. This increase was due to a direct effect of the drug and was not caused by salt restriction or changes in glomerular filtration rate. The rise in serum calcium is modest compared to the rise following the introduction of vitamin D and except for patients with mild hypoparathyroidism, thiazides are not an alternative to vitamin D. They may however reduce the oral calcium load required to maintain normocalcaemia.