Reactive arthritis is usually a transient acute arthritis appearing a few weeks after an infection somewhere in the body, mostly located in the urinary or the gastrointestinal tracts. It usually involves large joints, such as knees and ankles as well as the feet. There may be inflammatory changes located to the sacroiliac joints in the acute stage. The disease usually disappears after some months, but may recur and can develop into a chronic condition. This is especially the case in patients with the tissue type HLA B27. Patients with a chronic course often have involvement of the sacroiliac and/or spinal joints.