In a one-year double-blind study, the efficacy of hydrochlorothiazide was tested in 51 out-patients with recurrent calcium lithiasis. In the group to which thiazide had been administered, urinary calcium was significantly and lastingly reduced (p < 0.001 in fasting and daily urine), in the placebo group, however, it remained unaltered. Total serum calcium was unspecifically raised in both groups (protein-bound fraction) since ionized and ultrafiltrable calcium remained unchanged and parameters for parathyroid gland function tended to be reduced. The daily oxalate excretion in the urine was reduced in both groups during treatment and most expressedly in the placebo group (p < 0.05), which indicates an influence of unspecific factors. Activity products of calcium oxalate and brushite were reduced in both groups almost equally within the metastable region. In each group, 6 patients spontaneously developed kidney stones while treated. Our data indicate that effective reduction of urinary calcium by means of hydrochlorothiazide without dietary sodium restriction is possible and that causes other than urinary calcium must account for stone formation under thiazide administration.