Over twenty per cent of adult knees have a synovial plica which always has possibility of changes into pathologic synovial plica symptomes and signs resembling those of other common internal derangement of the knee joint. Especially the young active patient who has a history of trauma to the knee should once be suspected incarcerated synovial plica not only the possibility of internal derangement. Four cases of symptomatic synovial plica experienced by authors are reported with available references.