Anterior cruciate ligament (ACL) rupture usually leads to an altered stretch reflex excitability of the thigh muscles that stabilize the knee. The purpose of this study was to quantitatively assess reflex activity in the m. semitendinosus/semimembranosus after anterior tibial translation in 21 patients with isolated ACL ruptures. The patients were divided into a group with "giving way" symptoms (non-copers, n=12) and without "giving way" symptoms (copers, n=9). While the patients were standing upright with 30 degrees knee flexion, a force of 300 N was applied to the calf to induce anterior tibial translation. Activity of m. semitendinosus/semimembranosus was measured using surface electromyography (EMG). A linear potentiometer was placed on the tibial tuberosity and measured maximum tibial translation during standing, i.e. a functional condition. Knee laxity was assessed with a KT1000 arthrometer under passive conditions. After ACL rupture, the short latency response (SLR) latency remained unchanged (p=0.21), whereas for the medium latency response (MLR) it was significantly longer (p<0.001). Significantly longer MLR latencies were noted for non-copers compared to copers (p<0.01), while SLR latencies were similar. Significant differences between healthy and injured legs were noted after tibial translations using KT1000 (p<0.001) and during stance (p<0.001). Mechanical knee instability was found to be unchanged between copers and non-copers (KT1000: p=0.97; tibial translation: p=0.31). These results indicate that ACL rupture is associated with altered stretch reflex excitability which may lead to "giving way" symptoms, and that these changes may be more important for the development of "giving way" than the mechanical knee instability.