In order to clarify the effect of 1,25(OH)2D3 on the bone mineral content in senile osteoporosis, we examined the radial mineral density in 41 female cases of senile osteoporosis treated with 1,25(OH)2D3. The diagnostic criteria of senile osteoporosis were as follows. 1) Radial mineral density at below 0.5 g/cm2 by SPA 2) Bone dystrophy score in vertebra at over I degree 3) Presence of vertebral fracture 4) Over 60 years of age The subjects were divided into 5 groups: a control group (n = 11), a 0.25 microgram of 1,25(OH)2D3 once-a-day group (n = 5), a 0.5 microgram of 1,25(OH)2D3 once-a-day group (n = 8), a 0.25 microgram of 1,25(OH)2D3 twice-a-day group (n = 8) and a 0.25 microgram of 1,25(OH)2D3 three times-a-day group (n = 9). There was no significant difference in background data among these groups except for serum Al-P activity. The radial mineral density was measured in these 5 groups before and every 3 months after, starting the treatment by single photon absorptiometry in 1/3 distal site of radius for 1 year. No significant difference was detected in the serum levels of Ca, Pi and Al-P activity after starting the treatment among the 5 groups. The area under curve (AUC) of the radial mineral content after the treatment was calculated in each group. There was a significant dose-related increase in the AUC (p less than 0.05). However, the urinary Ca/Cr ratio was increased in the group receiving 0.75 microgram/day of 1,25(OH)2D3 The final AUC in the group receiving 0.5 microgram/day of 1,25(OH)2D3 tended to be very high compared with that in the other groups. From the above, it was suggested that 1,25(OH)2D3 might be effective for the treatment of senile osteoporosis especially at the dose of 0.5 microgram/day.