After developement of antimicrobial chemotherapy, morbidity and mortality from pyogenic arthritis has been reduced dramatically, but still this disease has remained as a serious and life threatening infectious disease of childhood or late sequelae in surviving patients. For the period of 6 years from January 1978 to December 1983, seventy nine patients, eigthy one cases who were treated as pyogenic arthritis at Severance Hospital and Yong Dong Hospital were studied retrospectively and the results are summerized as follows. 1. We thought that the main pathogeny of septic arthritis of knee joint are trauma and direct invasion of microorganism, because the frequency of pyogenic arthritis in knee joint are proportional to the age and history of accupuncture therapy and trauma are common. 2. In thirty three cases (40.7%) underlying causes were found, composed of infectious focus in eleven cases(13.6%) correspondent to hematologic transmission, osteomyelitis in seven cases(8.7%), trauma and accupuncture in six cases(7.4%) and eight cases(9.9%) corresponding to direct invasion. 3. In forty five cases(55.6%) microorganism are identified, among which coagulase positive staphylococci are in forty cases(88.9%) . 4. Tc-99m-MDP(Medronate) whole body bone scan were taken at twelve patients and positive findings were eleven cases. 5. In thirty eight cases(46.9%) the complications were appeared: Osteomyelitis of tibia in ankle joint and periarticular or subarticular bone defects in knee joints were common. 6. There were complications in every cases that the duration was more than eleven days, no exceptional.