The clinical details of 100 patients with proved acute appendicitis were compared with those of 100 patients with perforating or gangrenous appendicitis. Twenty features were found to be significantly different between the two groups. This information was incorporated into a computer data base and used in the differential diagnosis of abdominal pain. A program written to predict the probability that gangrene or perforation was present in patients with appendicitis gave a diagnostic accuracy over 91%. A clinical scoring index, which accurately predicted the state of the appendix in 88% of patients, was constructed from the significant differences between the two groups. When clinical scoring or computer analysis predicts a high probability of perforation or gangrene in patients with appendicitis, surgery should be performed without delay.