We present the case of a 53-y-old patient who complained of fever, pain and oedema of the right knee and the right calf. Colour ultrasonography showed thrombosis of the tibial and peroneal veins, while paracentesis of the knee joint showed characteristics of an inflammatory exudate. Culture of the arthritic fluid revealed Streptococcus pyogenes. Thus, a diagnosis of a septic arthritis of the right knee in association with deep venous thrombosis of the right lower leg was established. From a review of the literature we found no other cases of septic arthritis in association with thrombosis of the adjacent vein. In conclusion, in cases of septic arthritis of a joint one has to consider ruling out thrombosis of the adjacent venous vessels, especially when oedema is not confined to the joint. Missing the diagnosis of such a complication will lead to increased morbidity and mortality.