Nineteen adolescents with spastic diplegia and 12 able-bodied controls took part in a longitudinal study of aerobic capacity and physical working capacity during their teens. Absolute values for aerobic capacity and physical working capacity increased during the teens for both groups, which appears to contradict earlier descriptions of prematurely decreased physical working capacity among cerebral-palsied adolescents. On the other hand, net mechanical efficiency during submaximal bicycle ergometer tests decreased significantly in the diplegic group during their teens, and especially among those with severe motor handicap. This is of interest because these children are at risk of insufficient physical activity at school, partly because of the unsatisfactory design of physical education programmes for motor-handicapped children.