The aim of this study was to evaluate the accuracy of different methods of demonstrating right iliac fossa peritonism in appendicitis. The methods used were cat's eye symptom (pain on going over a bump in the road), cough sign, right iliac fossa tenderness, percussion tenderness, rebound tenderness and guarding. A series of 100 consecutive patients with a median age of 25 years (range 4-81 years), presenting with right iliac fossa pain were studied prospectively; the male:female ratio was 39:61. In all, 58 patients underwent operation, 44 had appendicitis confirmed on histology. Fourteen patients had a normal appendix removed; 11 were women aged between 16 and 45 years. Cat's eye symptom and cough sign were sensitive indicators of appendicitis (sensitivity 0.80 and 0.82, respectively), but were not specific (specificity 0.52 and 0.50, respectively) and therefore inaccurate (accuracy 64%). Percussion tenderness was less sensitive (sensitivity 0.57) but more specific (specificity 0.86). Rebound tenderness proved to be sensitive (sensitivity 0.82), specific (specificity 0.89) and accurate (accuracy 86%). Thus, rebound tenderness had a positive predictive value of 86% compared with 56% and 57% for cough sign and cat's eye symptom, respectively. In the difficult diagnostic group of young women, the positive predictive value of rebound tenderness was 88% compared with 58% and 56% for cat's eye symptom and cough sign. Appendicitis remains a difficult diagnosis, particularly in young women. Rebound tenderness still has an important role to play in clinical assessment.