With the aim of evaluating the role of 1,25 dihydroxyvitamin D in the pathogenesis of osteoporosis, this hormone was studied in 90 subjects. They were divided into three groups: the first group consisted of 30 normal female subjects (aged 30 to 45); the second group comprised 30 elderly female subjects (aged 67 to 90) the third group consisted of female patients (aged 65 to 87) with clinical and radiological evidence of osteoporosis. Variations of serum 1,25 dihydroxyvitamin D, after stimulation with oral P04 (1,000 mg/day for 10 days) and stimulation with oral Ca (1,000 mg/day for 10 days), were studied in 16 osteoporotic females (10 of them received P04) and six Ca (Controls). This second study was performed to evaluate the renal 1-alpha-hydroxylation. No significant variation between serum 1,25 dihydroxyvitamin D of elderly and osteoporotic females was observed when compared with normal young healthy subjects. Moreover, a significant increase in serum 1,25(OH)2D after oral P04 was found in accordance with a significant increase in serum parathormone. These data demonstrate that changes in serum 1,25(OH)2D levels did not seem to play a significant part in the pathogenesis of osteoporosis in our population neither could a deficit of renal 1-alpha-hydroxylation be demonstrated.