BACKGROUND Research investigating the most effective management of Achilles tendon injury has been limited by a lack of reliable outcome measurements. Calf strength may be a valid outcome measure, not only in terms of identifying possible risk factors for reoccurrence of rupture, but also as an indicator of recovery. Isokinetic dynamometry has been suggested as an effective tool for measuring the torque of the calf muscles. Such measurements have demonstrated high reliability for the assessment of calf muscle torque in healthy subjects. However, there are no published data to demonstrate the reliability of isokinetic dynamometry in subjects with pathology in the Achilles tendon. The purpose of this study was to assess the inter- and intraobserver reliability of isokinetic dynamometry for assessing plantarflexion torque following Achilles tendon rupture. METHODS Two independent observers used the Kin-com Dynamometer to measure the torque of the plantarflexors in 22 subjects 6 months after unilateral rupture of the Achilles tendon. Twelve subjects had been managed operatively and 10 nonoperatively. Subjects were placed in the prone position with the knee extended. Measurements of peak torque, average torque, and total work were made for both concentric and eccentric plantarflexion movements at 60 degrees per second. RESULTS Intraclass correlation coefficients were used to calculate reliability of measurements both within and between observers. Reliability was slightly greater on the healthy side (0.74-0.92 ICC) in comparison with the injured side (0.74-0.89 ICC). CONCLUSIONS The results of this study suggest that isokinetic dynamometry provides a reliable method of measuring the torque of the plantarflexors following rupture of the Achilles tendon, with levels of reliability comparable with those from healthy subjects. The study concludes that this would be a valuable and reliable outcome measure for use in clinical trials.