Geographical variations in blood pressure have been studied using an automatic sphygmomanometer in 2596 men and women aged 25-29, 40-44 and 55-59 living in nine British towns. In males aged 40-59, systolic blood pressure showed a range in age-adjusted town means of 9.0 mmHg (p less than 0.05); in females the difference of 8.6 mmHg was not significant (p = 0.14). Mean arterial pressure (MAP) and diastolic at age 40-59 were significantly different between towns for both sexes. Differences at age 25-29 were of a similar magnitude, and the mean town blood pressures at 25-29 correlated highly with those at 40-59 [systolic; males r = 0.74 (p less than 0.05), females r = 0.65 (p = 0.059)]. The ranking of town blood pressures in an earlier study was reflected in the present study, but stronger associations were observed with cardiovascular mortality. It is concluded that geographical blood pressure variations in Britain are established by age 25-29 years.