This study assessed the magnitude of changes in isokinetic muscle strength in children with juvenile idiopathic arthritis (JIA) before and after treatment with intra-articular corticosteroid injection and assessed the feasibility of a larger study of the same effect. Isokinetic dynamometry was used to measure peak knee extension and flexion torque in 12 children before and after treatment for unilateral knee arthritis. Extensor and flexor strength was reduced on the affected side before treatment (-0.56 Nm/kg, p = .004 and -0.24 Nm/kg, p = .02 respectively). Increases in extensor strength were observed at two weeks (p = .01) and twelve weeks postinjection (p = .03). Improvements at 6 weeks approached but did not reach statistical significance (p = .17). Improvements in flexor strength were not observed until 12 weeks postinjection (p = .03). Despite significant improvements in extensor strength, low peak knee extensor torque continued to be observed at 12 weeks (p = .01). Knee extensor and flexor strength is reduced in children with JIA with active arthritis and improves following intra-articular corticosteroid injection. Significant improvements in knee extensor and flexor strength were seen postinjection; however deficits in extensor strength were still evident at three months. Isokinetic dynamometry was safe and well tolerated in our sample of children with JIA with active arthritis.