Deficiency of circulating 1 alpha-25-dihydroxyvitamin D3 (1 alpha,25(OH)2D) regularly occurs in hypoparathyroidism (HP) and pseudohypoparathyroidism (PHP). Osteomalacia is occasionally found in the two diseases. Two patients, one with HP and the other with PHP, both with symptomatic and biopsy-proven osteomalacia, were studied before and after treatment with 1 alpha,25(OH)2D3. Laboratory values before treatment were as follows: serum immunoreactive parathyroid hormone was undetectable in the patient with HP and was elevated in the patient with PHP. Serum 25-hydroxyvitamin D, measured by binding assay, was 131.5 and 61.9 nmol/l (normal: 69.1 +/- 15.9 nmol/l); serum 24,25-dihydroxyvitamin D, measured by binding assay, was 13.9 and 3.8 nmol/l (normal: 3.4 +/- 1.4 nmol/l); serum 1 alpha,25(OH)2D, measured by bioassay, was 28.6 and 29.0 pmol/l (normal: 77.3 +/- 22.8 pmol/l) and, measured by receptor assay, was 36.2 and 41.0 pmol/l (normal: 71.8 +/- 35.8 pmol/l) in the HP and PHP patients, respectively. Serum calcium was low and serum inorganic phosphate was high in both cases. Treatment with 1 alpha,25(OH)2D3 (3-5 micrograms per day for 10-12 months) restored serum calcium and inorganic phosphate to normal, alleviated bone pain and healed the osteomalacia as shown on repeat bone biopsy. Our results provide further evidence that isolated deficiency of 1 alpha,25(OH)2D may cause osteomalacia or rickets.