Fourteen ambulatory children with spastic diplegia participated in a bilateral quadriceps strengthening program in an attempt to decrease the amount of knee crouch during gait. Each child exercised three times a week for six weeks using free ankle weights at a load of 65 per cent of maximum. A normal comparison group of 25 children was also tested under identical conditions. Children with cerebral palsy were significantly weaker in the quadriceps and hamstrings muscle groups than controls. Quadriceps strength increased significantly at all three angles of knee flexion as a result of the weight-training program and did not differ statistically from normal at the end of the program. Quadriceps weakness was shown to be a factor in crouch gait; restoring strength through resistance exercise may be a useful adjunct in the treatment of cerebral palsy.