Eighteen patients with 20 symptomatic hips underwent lengthening of the iliopsoas tendon for persistent painful snapping of this "internal" variety of snapping hip. We referred to the pathologic, painful snapping of the iliopsoas in the deep anterior groin as the "internal" snapping hip. This is in contrast to the more common and better-known "external" snapping that involves the greater trochanter and its overlying soft tissues. The results of our iliopsoas lengthening procedure are presented here. Lengthening of the iliopsoas tendon was accomplished by step cutting of the tendinous portion of the iliopsoas. The pathoanatomy of this poorly understood symptom complex was described in 1984 paper from this institution and is reviewed here. Iliopsoas bursography demonstrated a sudden jerking movement of the iliopsoas tendon between the anterior inferior iliac spine and iliopectineal eminence, synchronous with the patient's pain and often accompanied by an audible snap. The average preoperative duration of symptoms was 2.9 years, and the average length of postoperative followup was 25 months. All patients, except one, had a marked reduction in the frequency of snapping after tendon lengthening, and 14 of 20 hips had no snapping postoperatively. Of the six patients who had recurrence of snapping, all but one stated that this occurred much less frequently and was much less painful compared to the preoperative state. Two hips required reoperation. Postoperatively, only three patients complained of subjective weakness, and most patients were unlimited in physical activity with return to activities such as competitive football, pole vaulting, and long-distance running.(ABSTRACT TRUNCATED AT 250 WORDS)