Bone remodeling is controlled by systemic factors such as parathyroid hormone (PTH), calcitonin (CT), and 1,25(OH)2 vitamin D and by local factors including cytokines and growth factors such as IL-1, IL-2, TNF alpha, TGF beta, IFN alpha, and IFN gamma. Derangement of such control mechanisms leading to an imbalance between osteoclastic bone resorption and osteoblastic bone formation could cause osteoporosis. Conditions associated with immune dysfunction such as aging, corticosteroid therapy, and rheumatoid arthritis are associated with osteoporosis, which is also more common in females than in males, like most of the autoimmune-collagen diseases. Peripheral lymphocyte subsets CD4/CD8 were higher in patients with senile osteoporosis than in the age-matched controls, and returned to normal after 1 month of 1 alpha(OH)vitamin D3 treatment. On multiple regression analysis of histomorphometric data and lymphocyte subsets, a negative correlation was found between CD4 lymphocytes and bone resorption. High CD4 is thus associated with a low level of osteoclastic bone resorption or low turnover osteoporosis. Plasma interferon reflecting macrophage function decreased with advance in age and increased in response to 1 alpha(OH)D3 treatment. As one of the immunoregulators, vitamin D tends to stimulate the macrophage-natural killer system and suppress the lymphocyte system, stimulating TGF beta and TNF alpha activity. Senile osteoporosis of low turnover thus appears to be associated with vitamin D deficiency, low macrophage function, high CD4 lymphocyte proportion, low IL-1 and high IL-2 activity, low IFN alpha and high IFN gamma activity, and low TGF beta and TNF alpha activity. Treatment with vitamin D derivatives tends to reverse these changes.