In order to explore the possible immunological abnormalities as the background for the development of postmenopausal and senile osteoporosis, T lymphocyte subsets were identified by monoclonal antibodies of the Orthoclone OKT series. The OKT4/OKT8 (helper-inducer/suppressor T cell) ratio was significantly higher in osteoporotics with spinal compression fracture than in non-osteoporotics without such fracture in similar age. Total lymphocyte and T cell counts were similar between the two groups. Young control subjects had an OKT4/OKT8 ratio indistinguishable from elderly non-osteoporotics. Administration of 0.5 microgram/day 1 alpha-hydroxyvitamin D3 for 2 months resulted in a significant fall of the OKT4/OKT8 ratio in osteoporotics. While the significance of elevated OKT4/OKT8 in osteoporotics remains unknown, the fall of this ratio in response to administration of 1 alpha-hydroxyvitamin D3 suggests vitamin D dependence of this immunological abnormality associated with osteoporosis.