Hypervitaminosis D developed in four patients with osteoporosis or osteomalacia. All patients were given pharmacologic doses of vitamin D, had reduced baseline levels of renal function, and became hypercalcemic with acute renal failure. Measured 25-hydroxyvitamin D (25-OH D) levels were elevated in three patients; levels were not determined in a fourth patient who became normocalcemic when vitamin D therapy was discontinued. Published data on the use of vitamin D for prophylaxis or treatment of any form of osteoporosis fail to document benefits superior to those of calcium alone or calcium with estrogens and fluoride. Data on the use of 25-OH D show no greater benefit than for vitamin D. The use of 1,25-dihydroxyvitamin D (1,25-OH2 D) plus calcium may be superior to the use of calcium alone in some forms of osteoporosis. Vitamin D toxicity is associated with enhanced resorption of bone in some patients. Morbidity included extended hospitalization, dialysis, and chronic renal failure. Pharmacologic doses of vitamin D cannot be recommended for any form of osteoporosis.