To compare the efficacy of estriol (E3) in postmenopausal and senile osteoporosis, we administered orally 1 g/day calcium lactate either alone (control groups) or with 2 mg/day estriol (estrogen groups) for 10 months to 20 postmenopausal women aged 50-65 years and to 29 elderly women aged 70-84 years, and measured their bone mineral density of the lumbar vertebrae by dual energy X-ray absorptiometry. Out of 41 subjects who completed 10 months of treatment, 8 postmenopausal women and 12 elderly women in the estrogen groups had significant (p < 0.05) increases in bone mineral density (5.59 +/- 4.79% of the respective basal values). Ten postmenopausal women and 11 elderly women in the control groups had decreases bone mineral density (-4.02 +/- 7.00% and -3.26 +/- 4.60% of the respective basal values) at the 10th month. Genital bleeding as a side effect of estriol was seen in 6 out 29 elderly subjects at this dose. Moreover, decreases in the levels of calcium, total cholesterol, and triglycerides in serum, and an increase in the level of high-density lipoprotein-cholesterol were seen only in the elderly women receiving estriol. Although a lower dosage of estriol may be recommended for elderly subjects, these observations suggest that hormone replacement therapy with estriol is effective against degenerative osteoporosis, and that low-turnover bones in elderly women are also responsive to estriol.