.625mg/day of conjugated estrogen was administered to women after menopause for five years to investigate the preventive effect of HRT on postmenopausal bone loss. Bone mineral content of subjects before and after HRT was measured by the MD (microdensitometry) method. Subjects were: Group I: Those given HRT for the first time (52 cases); Group II: Those who had had HRT for more than 5 years and then were given another 5 years treatment (34 cases);
Control: Those with no HRT (26 cases). In the controls, the average level of sigma GS/D decreased significantly (p less than 0.005) from 2.465 to 2.209 in 5 years, while the average sigma GS/D level increased significantly (p less than 0.005) from 2.462 to 2.532 in Group I, and from 2.536 to 2.539 in Group II. Average bone mineral content after 5 years shown by the % based on the initial bone mineral content as 100% decreased to 90.72% (9.28% decrease) in the controls, but increased to 102.91% in Group I, and 100.26% in Group II. The increase ratios were significantly (p less than 0.005) different in Groups I and II. Thus, HRT was seen to be very effective in preventing bone loss after menopause. The effect in Group II was weaker than that in Group I, but much stronger than in the controls.