It has been suggested that a hyperkinetic circulatory state, with high cardiac output, causes high blood pressure in childhood and, secondarily, in adulthood. We studied blood pressure and cardiac output in 319 subjects aged 15 to 19. Blood pressure was measured with an automated device; cardiac output was estimated by M-mode echocardiography and indexed by body surface area. The distribution of cardiac output was stratified using quartiles. Mean arterial pressure was virtually constant over these strata, with boys and girls showing essentially the same pattern. Linear regression of mean arterial pressure on cardiac output yielded a coefficient which was not significantly different from zero. A history of high blood pressure in the parents was positively associated with mean arterial pressure, but unrelated to cardiac output, in the offspring. This evidence does not support the hypothesis that the hyperkinetic circulatory state causes high blood pressure in childhood; rather, raised blood pressure in adolescents appears to relate to increased peripheral vascular resistance. Therefore, our findings lend support to the view that change in blood pressure over time is caused by a gradual increase in peripheral resistance beginning early in life.