Lyme disease, a multi-system disorder may be associated with arthritis. Lyme arthritis most commonly affects the knee joints. Ultrasonography can show the inflammation changes of the knee joint and can be a usefull method in diagnosis of Lyme arthritis. The most freguent ultrasonographic finding was knee joint effusion. Because of Lyme arthritis similarities to rheumatoid arthritis, a serologic test antibodies against cyclic cytrulinated peptid (anty CCP) can be helpfull in distinguishing of these two diseases.