Serum 1, 25-dihydroxyvitamin D [1, 25 (OH) ₂D], parathyroid hormone (PTH), calcitonin (CT), calcium, inorganic phosphorus and alkaline phosphatase (Al-P'ase) levels were determined in 24 patients with senile or postmenopausal osteoporosis, which was diagnosed by lateral X-ray film of lumbar vertebrae and divided into 3 stages, porosis score I, II and III according to its severity. Serum calcium in osteoporotic group with porosis score II or III was within normal limits but significantly increased compared to that of the age-matched normal group. There was no significant difference in serum inorganic phosphorus and serum Al-P'ase levels between osteoporotic groups and age-matched normal group. Serum 1, 25 (OH) ₂D level determined by radioreceptor assay was significantly decreased in the osteoporotic group with porosis score III compared to that of normal group. Serum PTH was supernomal or higher than normal level in all of osteoporotic groups, while serum CT was within normal limits in those osteoporotic groups. There was no significant correlation among serum 1, 25 (OH) ₂D, PTH and CT or between any of these calcium-regulating hormones and serum calcium, inorganic phosphorus and Al-P'ase, either in osteoporotic groups or in normal group. From these results, it is presumable that a decrease in serum 1, 25 (OH) ₂D is not the only factor for the pathogenesis of senile osteoporosis, and some abnormality in the receptors of the target organs to 1, 25 (OH) ₂D₃ or some other factor than 1, 25 (OH) ₂D₃ might be playing a role for an increase of serum PTH level leading to an increase of bone resorption.