Individual studies of stroke have not clearly answered two questions: on the relation, if any, between total blood cholesterol and stroke; and on how the strength of the relation between diastolic blood pressure and stroke varies with age. The associations of blood cholesterol and diastolic blood pressure with subsequent stroke rates were investigated by review of 45 prospective observational cohorts involving 450,000 individuals with 5-30 years of follow-up (mean 16 years, total 7.3 million person-years of observation), during which 13,397 participants were recorded as having had a stroke. Most of these were fatal strokes in studies that recorded only mortality and not incidence, but about one-quarter were from studies that recorded both fatal and non-fatal strokes. After standardization for age, there was no association between blood cholesterol and stroke except, perhaps, in those under 45 years of age when screened. This lack of association was not influenced by adjustment for sex, diastolic blood pressure, history of coronary disease, or ethnicity (Asian or non-Asian). However, because the types of the strokes were not centrally available, the lack of any overall relation might conceal a positive association with ischaemic stroke together with a negative association with haemorrhagic stroke. When the highest and the lowest of the six blood pressure categories were compared, the difference in usual diastolic blood pressure was 27 mm Hg (102 vs 75 mm Hg), and there was a fivefold difference in stroke risk. This fivefold difference was seen both in those with a pre-existing history of coronary heart disease and in those without it. The proportional difference in stroke risk, however, was more extreme in middle than in old age. Among those aged < 45, 45-64, and 65+ when screened, the differences in the relative risks of stroke (between the highest diastolic blood pressure category and a combination of the lowest two categories) were tenfold, fivefold, and twofold, respectively. However, because the absolute stroke risks are greater in old age, the absolute differences in the annual stroke rates showed an opposite pattern, being 2, 5, and 8 per thousand, respectively. This suggests that the effects of therapeutic blood pressure reductions should be assessed separately in middle age and in old age.