Early and accurate diagnosis of traumatic disorders of the knee joint is mandatory for the proper treatment and good results. There are many diagnostic methods for the ligamentous instability, but stress test seems to be still most widely used. Manual test gives us primitive and very valuable information, but it is subjective and lacks reproducibility. In contrast, arthrometer affords higher reproducibility and quantification than manual test, hence it is also a useful method for diagnosis and post operative follow up. Authors reviewed and compared the diagnostic method of 56 cases of acute ligamentous injury which was confirmed by exploration, using arthrometer and manual stress test from October 1990 to May 1992 at Asan Medical Center. The results are as follows; 1. Sensitivities in isolated collateral ligament rupture proved to be 42.6% by arthrometric test and 83.3% by manual stress test. 2. Sensitivities in collateral ligament rupture combined with cruciate ligament rupture proved be 57.1% by arthrometric test and 88.2% by manual stress test. 3. Sensitivities in isolated cruciate ligament rupture proved to be 62.5% by arthrometric test and 85.7% by manual stress test. Stress test by arthrometer has merits that protect examiner from radiation exposure and show high reproducibility through quantification. On the contrary, it was proved that false negative ratio is higher than manual test in isolated injury, byt similar results in combined injury.