Piriformis Syndrome is characterized by pain of the low back, groin, perineum, buttock, hip, posterior thigh, leg and foot. Symptoms are aggravated by sitting, prolonged combination of hip flexion, adduction, the medial rotation, or by activity. In addition, patient may complain of painful swelling of the limb and sexual dysfunction-dyspareunia in female, and impotence in male. It currently appears that three specific conditions may contribute to the piriformis syndrome: (a) myofascial pain; (b) nerve and vascular entrapment; (c) dysfunction of the sacroiliac joint. The important keys of diagnosis are history and physical examination. There is no known objective diagnostic method. We described the clinical features of four cases of piriformis syndrome and reviewed foreign literature.