We examined the relationship between age at natural menopause and mortality of ischemic heart disease in 19,309 Norwegian postmenopausal women. A total of 2767 fatal infarctions occurred during 29 years of follow up. Overall, a relatively weak inverse relationship was seen with approximately 10% lower ischemic heart disease mortality in women aged > or = 47 years at the menopause compared to women with an early menopause (< 44 years). Risk estimates were similar for women aged 47 and more at menopause. However, the inverse relationship was stronger and statistically significant (p = 0.01) in women aged less than 70 years. In this group of women, we observed a nearly 60% reduction in the ischemic heart disease mortality in women with a late menopause (> or = 53 years) compared to women aged < 44 years at menopause (mortality rate ratio = 0.42; 95% confidence interval 0.25-0.72). This protective effect of a late menopause is reduced with advancing age, however, and is of minor significance in the age groups where the great proportion of the ischemic heart disease deaths occur.