Pyogenic arthritis poses a serious complication to the opening of a joint. In the past eight years we have treated 22 patients suffering from acute infection of the knee joint and the talo-crural joint. Cases with infection of osseous origin were excluded. Infection was mainly caused by contamination during ligament reconstruction. Our therapy consisted of emergency operative revision of the joint, removal of necrotic tissue, and installation of an irrigation-suction-drainage system. The joints were immobilized and systemic antibiotics were administered in all cases. In four cases the installation of the irrigation-suction-drainage system in the knee joint was performed percutaneously without opening the joint. Of these, three had to be reoperated as the infection could not be controlled. During recent years we preferred to use a 0.5% PVP-I-solution for irrigation, because of the antiseptic effect. Irrigation treatment is limited to a period of seven days. Two out of 15 cases of knee-joint infection and three cases out of seven of talo-crural-joint infection resulted in ankylosis. In cases in which treatment began in the first week after contamination almost all had excellent clinical results. Substantial previous damage to the joint or delayed treatment usually prevented complete restoration of the joint's function.