Recurrent anterior and interal rotatory subluxation of the tibial plateau is a disabling symptom in patients with insufficiency of the anterior cruciate ligament. Since 1970 we have used the N-test, a special jolt test to detect the characteristic instability. One hundred eighteen cases with anterior-internal rotatory instability of the knee seen at the Kanto Rosai Hospital and the University of Tokyo Hospital and diagnosed by arthroscopy and arthrotomy from 1973 to 1978 were reviewed. Most of the patients were athletes or sportsmen and the injuries commonly occurred in deceleration-injury mechanism and 81% of the cases were associated with meniscal tear. Thirty-two cases were reconstructed by iliotibial tract transfer, twenty of whom had been evaluated from 1-5 years after operation. Postoperatively, giving-way incidents disappeared in eleven, markedly decreased in five, and were unchanged in four patients. The anterior and internal rotatory subluxation of the tibial plateau disappeared or decreased in seventeen and was unchanged in two cases. Some type of defective anterior cruciate ligament was recognized in all our 118 cases and the capsular lesions were considered to play only a secondary role in exhibiting the instability.