To analyse the pattern of clinical presentation of perforated and non-perforated acute appendicitis in 544 unselected patients treated surgically for suspected acute appendicitis; to find out if their presentation differed. Open prospective population-based study. Teaching hospital, western Norway. All 434 patients operated on for suspected acute appendicitis from the catchment area of a single hospital in whom the diagnosis was confirmed. Appendicectomy; history, clinical findings, and inflammatory markers were recorded in all patients. Odds Ratio (OR) and 95% confidence interval (CI) obtained by logistic regression analysis of symptoms and signs of acute appendicitis with regard to independent predictors of perforation of the appendix. Receiver operating characteristic (ROC) curve analysis of inflammatory markers. The perforation rate was 20% (n = 88). Perforation was unlikely when abdominal pain was limited to the right iliac fossa, (OR 0.13, 95% CI 0.05 to 0.33). Increased C-reactive protein (CRP) concentration >50 U/L (OR 4.6, 95% Cl 2.44 to 8.75) and greater age (by decade; OR 1.18, 95% CI 1.02 to 1.36) were independent predictors of perforation of the appendix. Clinical differences between perforated and non-perforated acute appendicitis could be explained by the presence of advanced inflammation in patients with perforated appendicitis. Our data do not support the hypothesis that they are two clinically different diseases.