In connection with two observations histologically proved on a metastasic territory of osteomalacia appeared during osteocondensing metastases of prostatic origin, the authors underline the responsibility of sudden increase of osteoformation in the hypocalcemia genesis and put the emphasis on the transitory aggravating function of the given high doses of estrogen within negative calcium balance. A D vitamin deficiency, frequent among old patients, could increase osseous manifestation of calcium supply insufficiency responsible of an authentic osteomalacia. This deficiency is not secondary to a phosphorus diabetes and is amended by an important calciu addition and by the administration of D vitamin metabolites as proved by repeated osseous biopsy with dynamic and histomorphometric studies.