This paper reports a controlled trial of human and computer-aided diagnosis in a series of 552 patients with acute abdominal pain. The overall diagnostic accuracy of the computer-aided system was 91·5% and that of the senior clinician to see each case was 81·2%. However, the clinician's diagnostic performance improved markedly during the period of the trial. The proportion of appendices which perforated before operation fell from 36% to 4% during the trial, and the negative laparotomy rate dropped sharply. After the trial closed in August 1972 these figures reverted towards their pretrial levels. It is suggested that while computer-aided diagnosis is a valuable direct adjunct to the clinician dealing with the “acute abdomen,” he may also benefit in the short-term from the constant feedback he receives and from the disciplines and constraints involved in communicating with the computer.