Bone pains observed in patients undergoing estrogen therapy, and presenting with osteoblastic metastases from prostatic cancer are usually related to unsuccessful treatment. In some patients, these pains may result from osteomalacia--ie incomplete mineralization of the new bone--because of the drainage of calcium by the osteoblastic metastases. A clinical, biological and histomorphometric study of bone specimens without decalcification was conducted in ten patients with osteoblastic disease secondary to prostatic carcinoma, who were under estrogen therapy, and for whom a change of therapy was contemplated. The study reports three cases of osteomalacia. Their bone pains were more intense, more diffuse and more permanent than those registered by patients without osteomalacia. All three had had previous fractures of the neck of the femur and a low urinary and serum calcium and phosphorus content. The discovery of osteomalacia by histomorphometric study is important because it allows effective, etiological treatment of the bone pains in these patients.