In an effort to maintain normal serum calcium levels without inducing hypercalciuria, we treated seven hypoparathyroid patients for up to 25 months with chlorthalidone, a thiazide-like sulfonamide diuretic, plus a salt-restricted diet, without added vitamin D. Mean 24-hour calcium excretion decreased from 179 to 88 mg (P less than 0.001), and mean serum calcium increased from 8.2 to 9.3 mg per deciliter (P less than 0.05). Diuretic therapy or moderate salt restriction alone was not as effective as combined therapy. Beneficial effects were sustained for as long as therapy was maintained. The rise in serum calcium, which involves the filterable and ionized fractions, cannot be due entirely to reduced excretion and may in part be explained by increased intestinal absorption. Oral chlorthalidone plus a low salt diet appears to be an effective alternative to vitamin D in the maintenance therapy of at least some patients with hypoparathyroidism.