During maximal dynamic exercise the blood pressure (BP) was measured in 497 healthy 9- to 18-year-old children. Systolic BP increased more in the postpubertal groups than in the prepubertal ones. It was also higher in the boys than in the girls of the same age. This was due to a higher work load in boys than girls. Twenty-two subjects had a systolic BP of 200 mmHg or more during the exercise. Only 2 had a resting systolic BP exceeding the mean by 2 standard deviations or more. Three postpubertal boys reached a systolic BP of 240 mmHg at heart rate 170. None had an elevated resting BP. It may be concluded that it cannot be predicted on the basis of the resting BP whether or not an individual is going to have an excessive increase in systolic BP during exercise. The increase in systolic BP to dangerous levels, e.g. 240 mmHg or more, during exercise can only be excluded by means of an individual exercise test.