The histological and biochemical response of osteomalacia has been studied in four patients with primary biliary cirrhosis, who were treated with oral 25-hydroxyvitamin D3, 50 microgram daily, or intramuscular vitamin D2, 150,000 units once weekly, for five to 12 months. All patients showed complete histological healing of osteomalacia, despite rapidly deteriorating liver function in three. Plasma 25-hydroxyvitamin D concentrations were low in all patients before treatment, but became normal during either vitamin therapy. Serum calcium and phosphate levels, and urinary calcium excretion were not always reliable in predicting the histological response to treatment. Serum alkaline phosphatase activity decreased in all patients during vitamin D therapy. We conclude that both high-dose parenteral vitamin D2 and oral 25-hydroxyvitamin D3 may be effective in healing osteomalacia associated with primary biliary cirrhosis. Measurement of plasma 25-hydroxyvitamin D levels during vitamin D therapy provides useful information about 25-hydroxylation of the parent vitamin and intestinal absorption of orally administered 25-hydroxyvitamin D3.