We studied the feasibility, reliability, and reproducibility of an "all-out" 30-second cycling or arm cranking test of muscle power and muscle endurance (the Wingate Anaerobic Test) in 66 girls and boys aged 5 to 18 years old who had cerebral palsy or a myopathy. The arm and leg tests, given in duplicate, were feasible in 94% and 61% of subjects, respectively. Reliability coefficients exceeded .95 in patients with spastic cerebral palsy and myopathies and were somewhat lower in those with athetotic cerebral palsy. Means and SDs were similar in both trials. We conclude that the Wingate Anaerobic Test is feasible, highly reliable, and reproducible in these patients.