The hamstrings:quadriceps muscle strength ratio has been used as an indicator of normal balance between the knee flexors and extensors. A more functional approach to this strength ratio would be to compare opposite muscle actions of antagonistic muscle groups. The dynamic strength control ratio (DSCR) should give a more appropriate measure relating to knee function. There is a lack of normative data relating to DSCR for anterior cruciate ligament (ACL) deficient subjects. Effects of ACL deficiency on isokinetic peak torque for eccentric and concentric muscle actions of the quadriceps and hamstrings, in conjunction with isometric peak torque, were examined in 10 patients awaiting reconstructive surgery (male = 8, female = 2 ; age = 32.8 +/- 8.3 years; height = 1.77 +/- 0.08 m; mass = 72.1 +/- 12.5 kg). These variables were assessed using an isokinetic dynamometer. The results were considered in terms of the conventional ratio and DSCR. Anterior tibial drawer was measured using a knee ligament arthrometer to confirm clinical diagnosis of ACL rupture. The isokinetic peak torque data analysed were for angular velocities of 1.05 rad s(-1) (60 degrees s(-1)). Significant strength deficits were apparent between normal and injured sides for: concentric isokinetic quadriceps action (p < 0.05); isometric quadriceps action at 70 degrees of knee flexion (p < 0.05); isometric quadriceps action at 40 degrees of knee flexion (p<0.01); eccentric isokinetic hamstrings action (p < 0.05). With bilateral comparison, the conventional strength ratios showed no significant difference, as did the DSCR. The bilateral comparison of isometric strength ratios revealed significant losses in quadriceps strength for the injured side (p < 0.05) but no significant losses in hamstring strength (p > 0.05). Thus, differences can be seen in conventional ratios and DSCR for ACL-deficient subjects. This is an area of clinical interest with the increasing frequency of ACL reconstruction using hamstrings tendons.