A prospective study of 106 patients who had static interlocking nailing of the shaft of the femur was performed to determine the relationship between the duration and magnitude of intraoperative traction and the development of a pudendal nerve palsy. A strain-gauge, mounted in the countertraction post, measured the magnitude of the perineal pressure over time. All nailings were performed with the patient in the supine position. Postoperatively, the patients were interviewed by one of us, who had been blinded from the results of the recordings of intraoperative pressure, for a history of erectile dysfunction and changes in labial, scrotal, or penile sensation. A light-touch sensory examination of the genitalia was performed on all patients. Ten patients (six men and four women) had a pudendal nerve palsy: nine had sensory changes only, and one complained of erectile dysfunction. The symptoms had resolved at the three-month follow-up evaluation in all patients except one man who complained of dysesthesia six months postoperatively. The patients in whom a palsy did not develop had been positioned on the fracture-table and the perineal post for an average of 2.6 hours (range, 1.4 to 5.2 hours) compared with an average of 2.8 hours (range, 2.0 to 4.3 hours) for those in whom a palsy did not develop (p = 0.15). The magnitude of the total traction forces averaged 34.9 kilogram-hours for the patients who did not have a palsy compared with 73.3 kilogram-hours for those who did (p < 0.03). Adduction of the hip, as well as manipulations for reduction of the fracture, significantly increased the traction forces.(ABSTRACT TRUNCATED AT 250 WORDS)