Transiliac undecalcified bone biopsy specimens were taken after tetracycline double labeling from 14 patients with radiologic evidence of osteosclerotic metastases from prostatic carcinoma. The histomorphometric analysis showed an increased trabecular bone volume in all patients, and in seven morphologic and dynamic evidence of osteomalacia (Group 1). The seven other patients demonstrated an extension of apposition surfaces without evidence of osteomalacia (Group 2). Group 1 was different from Group 2 in terms of a greater increase in serum alkaline phosphatase and a lower urinary calcium. In four Group 1 patients, a second bone sample taken after two to six months of treatment with vitamin D and calcium provided evidence of improving osteomalacia. The high incidence of osteomalacia in osteosclerotic metastases of prostatic origin appears to be the result of the increase in bone formation induced by prostatic cells, and the unability to satisfy the high calcium demand for new bone.