OBJECTIVE: The purpose of this study was to examine age-related differences in reasons that postmenopausal women began and stopped hormone replacement therapy (HRT). DESIGN: Two identical telephone surveys were conducted of women members of Kaiser Foundation Health Plan who had begun HRT within the previous 3 years. The first, in 1997, was of 604 older women aged 65 years or older; the second, in 1998, was of 866 younger women aged 50-55 years. Prescription records for both groups provided the means for determining continuation of therapy. RESULTS: Among older women, 35% reported prevention or treatment of osteoporosis as the primary reason for starting HRT. Younger women were less likely (14%) to report this (p < 0.001). Relief of vasomotor menopausal symptoms was the most frequently reported reason that younger women gave for starting HRT; it was the primary reason in 34%. In contrast, only 7% of older women reported relief of vasomotor symptoms as the primary reason for starting HRT (p < 0.001). Older women were more likely than younger women to discontinue HRT; after 12 months, the probabilities of discontinuation were 62% and 48% (relative risk = 1.4; 95% confidence interval = 1.2-1.6). Treatment-related side effects were most often the reason given for stopping HRT; 87% of older women and 64% of younger women who stopped reported that a treatment side effect was their primary reason (p < 0.001). Among treatment side effects, vaginal bleeding was the most frequently reported reason for stopping HRT; it was the primary reason for stopping in 52% of older women and 29% of younger women (p < 0.001). CONCLUSIONS: Older women differ from younger women in their reasons for starting and stopping HRT. Whereas osteoporosis is the predominant reason that older women begin HRT, relief of vasomotor symptoms is the major reason that younger women begin. Early discontinuation of HRT is common and is greater among older women. Intolerance of treatment, particularly vaginal bleeding, is the predominant reason for stopping HRT.