In order to examine the effects of vitamin D on osteoblast function and to evaluate if osteoporotic and normal osteoblasts show a different behaviour in response to vitamin D, this report investigates the changes in osteocalcin production, after 1,25-dihydroxy-vitamin D(3) stimulation of cultured osteoblasts derived from osteoporotic patients. Our results indicate an inadequate osteoblastic function in osteoporosis and demostrate that 1,25-dihydroxy-vitamin D(3) can stimulate the metabolic activity of human osteoblasts in vitro. Considering that osteoporotic bone samples were representative of senile osteoporosis, our results may indicate a different metabolic phenotype in osteoporotic osteoblasts compared with normal osteoblasts. The increased osteocalcin production after 1,25-dihydroxy-vitamin D(3) stimulation of osteoporotic osteoblasts suggests a reduced, but not absent, anabolic function in senile osteoporotic osteoblasts. The results of this study confirm the validity of vitamin D(3) to treat senile osteoporosis and suggest the need of higher vitamin D(3) intake in senile osteoporotic patients than in younger subjects.