The J-shaped relation between diastolic blood pressure and mortality from coronary heart disease continues to provoke controversy. We examined the association between diastolic blood pressure and progression of aortic atherosclerosis in a population-based cohort of 855 women, aged 45-64 years at baseline. The women were examined radiographically for calcified deposits in the abdominal aorta, which have been shown to reflect intimal atherosclerosis. After 9 years of follow-up, slight progression of atherosclerosis was noted in 19% of women and substantial progression in 16%. The age-adjusted relative risk of substantial atherosclerotic progression in women with a decrease in diastolic pressure of 10 mm Hg or more was 2.5 (95% CI 1.3-5.6), compared with the reference group of women who had a smaller decrease or no change. The excess risk in this group was confined to women whose increase in pulse pressure was above the median (3.9 [1.5-9.9] vs 1.1 [0.3-4.2] in women with an increase in pulse pressure below the median). The relative risks for women with rises in diastolic pressure of 1-9 mm Hg and 10 mm Hg or more were 2.2 (1.1-4.3) and 3.5 (1.6-8.0), respectively. These findings suggest that a decline in diastolic blood pressure indicates vessel wall stiffening associated with atherosclerotic progression. They support the hypothesis that in low-risk subjects progression of atherosclerosis may be accompanied by a decrease in diastolic blood pressure rather than the opposing idea that low diastolic blood pressure precipitates the occurrence of atherosclerotic events.