Arthroscopy has now become a valuable method for the diagnosis and treatment of the internal derangement of the knee. Based on the experiences gained during the past three years, in which 93 knees with posttraumatic disorders were observed arthroscopically, several uncommon and interesting findings were encountered. Such illustrative findings have been summarized here. I. Injury at the parameniscal portion is readily overlooked and especially the middle and posterior one-thirds are inaccessible. A wavy undulation of the concave margin of the meniscus, when abnormally exaggera;ed by tibial rotation, possibly suggests the presence of this type of injury. 2. The discoid cartilage and the bucket-handle tear of meniscus occasionally show similar pattern arthroscopically. 3. The plica of the knee can sometimes be a cause of knee disorders. Any internal derangement, which tends to be localized around the patella, warrants close scrutiny for its presence. 4. A large foreign body within a knee joint could easily be removed by virtue of direct arthroscopic visualization.