The authors report their experience of the arthroscopic diagnosis of acute traumatic hemarthrosis of the knee. They point out the value of testing stability under anaesthesia. A very important laxity, according to severe damage of peripheric structures and cruciate ligaments, need surgical repair. Otherwise, in all cases with minor instability, arthroscopy is very useful: some arthrotomies are so avoided, some associated lesions were diagnosed. Any way arthroscopy is reported to allow adequate treatment of traumatic hemarthrosis of the knee.