Test-retest reliability of measurements of peak aerobic power (cycle ergometer), anaerobic power (cycle ergometer), and isokinetic muscle strength of the knee (Cybex) was established in 12 young children with cerebral palsy (spastic diplegia/tetraplegia; mean age 8.8 years) and in addition in 39 healthy controls (mean age 9.2 years). The cycle ergometer tests were found to be reliable in both the group with CP and the control group (test-retest correlations varying from 0.72 to 0.96). The isokinetic strength test in the group with CP was only reliable at 30 degrees/s, whereas in the control group high test-retest correlations were also found at 60 degrees/s and 120 degrees/s.