Diagnostic arthroscopy including the use of a probe should be performed if the cause for a haemarthrosis of the knee cannot be established by radiological means. The clinical examination of the recently injured knee must be considered to be unreliable due to pain. In approximately one-third of knee injuries an incorrect or incomplete diagnosis is made. Even if ligamentous injuries can be established, arthroscopy is mandatory, because this is the only sure way in which acute and degenerative meniscal or cartilaginous lesions can be diagnosed. Arthroscopy should be performed under general or spinal anaesthesia to allow the use of a tourniquet and the immediate repair of ligaments if deemed necessary. The experienced arthroscopist needs only a few minutes to obtain information that could influence the approach to the injury and its longterm prognosis. In a series of 1238 arthroscopies 252 (20.3%) were done for haemarthrosis of unknown origin. In 68% of these cases diagnosis could only have been established by arthroscopy. In this series of 252 cases 23% had an isolated complete anterior cruciate ligament (ACL) injury, 28% had anterior-medical instability, 8% had partial ACL lesions, 5% posterior cruciate ligament injuries, 15% medial collateral ligament injuries, 11% had only synovial tears or contusions. 5% of cases had chronic ACL instability, 4% had osteochondral fragments which could not be diagnosed radiologically. A recent traumatic patellar dislocation with a tear of the retinaculum was found in 1.6% of the cases.(ABSTRACT TRUNCATED AT 250 WORDS)