The irritable bowel syndrome is the commonest diagnosis in gastroenterological clinics, although diagnostic criteria and investigatory programs vary. To elucidate the diagnostic safety and prognosis of the syndrome, a retrospective study was conducted. One hundred and twelve consecutive patients with irritable bowel syndrome as the final and only abdominal diagnosis in the period 1977-79 were followed up in 1984. Seventeen patients died during the follow-up period; two of these were considered diagnostic failures (chronic pancreatitis and pancreatic cancer). Of the remaining 95 patients, 93 were available for the follow-up study. Three diagnostic failures were found (gallbladder stones, kidney stone, thyrotoxicosis). The diagnostic failure rate was accordingly 4.5% (5/110). Half of the patients had unchanged or aggravated symptoms at the follow-up study, independent of treatment. The only predictor of a poor prognosis was abdominal surgery before the diagnosis.