This paper reports a survey of human and computer-aided diagnosis in a prospective consecutive series of 301 patients admitted to the hospital with lower gastrointestinal tract disease. At initial outpatient contact (at which time endoscopy was customarily performed), the clinicians' diagnostic accuracy was 64.5%. After biopsy, radiology, and other investigative procedures, the clinicians' preoperative diagnostic accuracy rose t0 82.7%. When data from the house surgeon's case notes were fed into the computer, its diagnostic prediction proved accurate in 77% of the 301 patients, and when details of the outpatient endoscopy were added to the house surgeon's case notes, the computer's diagnostic accuracy rose to 84.7%. It is suggested that computer-aided analysis may have a limited part to play in discriminating between the common causes of lower gastrointestinal disorders in routine clinical practice.