A special instrument was developed in order to quantify intraoperatively the traction forces which are employed during Shouldice repair of a primary inguinal hernia while adapting the lateral edge of the rectus sheath and the iliopubic tract. Further investigations were performed under the Valsalva manoeuvre and simultaneous measurement of the intra-abdominal pressure. In 25 patients an average increase in traction forces of 3.62 +/- 0.60 N was registered when the hernial gap was decreased to 0.3 cm. By applying the Valsalva manoeuvre with an average increase of 32 mm Hg in intra-abdominal pressure, traction was increased by a further 1.67 +/- 0.20 N on average. Hernia repair, tensing of the abdominal muscles and increasing the intra-abdominal pressure, however, have a small effect on the traction forces affecting the edges of the hernial gap. It may therefore be concluded that these factors, frequently believed causal for early hernia recurrence, are actually less influential as the force vector stressing the repair zone points in a different direction.