Seventy-four patients were operated on by arthroscopic synovectomy after being diagnosed with symptomatic localized post-traumatic synovitis in a 5-year period. Sixty-six patients (69 knees) could be evaluated at the end of the follow-up. The indication for surgery was severe mechanical type pain (score 3) after an evident injury nonresponding to 3 months of conservative treatment. Any other source of pain was preoperatively excluded by means of radiographs and magnetic resonance imaging, and intraoperatively by means of arthroscopic examination. The results were assessed according to a pain scale (score 0-3). Preoperatively all patients had severe pain (score 3). After a mean follow-up of 2.8 years (range, 1 to 6 years) there were 43 patients without pain (score 0), 15 with mild pain (score 1), 8 with moderate pain (score 2), and 3 with severe pain (score 3). In summary, this retrospective study appears to show that arthroscopic synovectomy should be taken into account when facing patients with painful localized post-traumatic synovitis of the knee joint. This is a diagnosis of exclusion as other internal derangements have been eliminated by magnetic resonance imaging and arthroscopic examination.