It is known that cerebral blood flow declines with age in sedentary adults, although previous studies have involved small sample sizes, making the exact estimate of decline imprecise and the effects of possible moderator variables unknown. Animal studies indicate that aerobic exercise can elevate cerebral blood flow; however, this possibility has not been examined in humans. We examined how regular aerobic exercise affects the age-related decline in blood flow velocity in the middle cerebral artery (MCAv) in healthy humans. Maximal oxygen consumption, body mass index (BMI), blood pressure and MCAv were measured in healthy sedentary (n = 153) and endurance-trained (n = 154) men aged between 18 and 79 years. The relationships between age, training status, BMI and MCAv were examined using analysis of covariance methods. Mean +/- s.e.m. estimates of regression coefficients and 95% confidence intervals (95% CI) were calculated. The age-related decline in MCAv was -0.76 +/- 0.04 cm s(-1) year(-1) (95% CI = -0.69 to -0.83, r(2) = 0.66, P < 0.0005) and was independent of training status (P = 0.65). Nevertheless, MCAv was consistently elevated by 9.1 +/- 3.3 cm s(-1) (CI = 2.7-15.6, P = 0.006) in endurance-trained men throughout the age range. This approximately 17% difference between trained and sedentary men amounted to an approximate 10 year reduction in MCAv 'age' and was robust to between-group differences in BMI and blood pressure. Regular aerobic-endurance exercise is associated with higher MCAv in men aged 18-79 years. The persistence of this finding in older endurance-trained men may therefore help explain why there is a lower risk of cerebrovascular disease in this population.