In a series of 356 arthroscopies of the knee joint 127 patients were explored by arthrotomy. A Storz arthroscope was introduced through the patellar tendon, and we alternated between the 30 degrees and 70 degrees optical systems in the same trocar sheath. The 30 degrees system was used for inspection of the superior, anterior, medial and lateral compartments, and to lead the tip of the instrument to the posterior part of the joint. The 70 degrees telescope was then used, providing a good view of the posterior cruciate ligament and the posterior horn of the menisci and their attachments. It also allowed direct inspection of the posterolateral and posteromedial compartments. No major diagnostic error was made by the arthroscopist in the 127 patients operated on as a result of the findings of endoscopic examination. Arthroscopy with the use of both 30 degrees and 70 degrees telescopes at the same session gives high diagnostic accuracy and detailed, exact pre-operative diagnosis.