An analysis has been made of 235 deaths that occurred among 1905 patients with peptic ulcer who constituted a random sample of the occurrence of ulcer disease in an area of Denmark comprising half a million inhabitants. The disease itself, according to the death certificate, was considered the primary cause of death in 10% of the cases; half of these had been operated on immediately before death. The other patients died more frequently than expected from the following causes: chronic bronchitis, pulmonary emphysema, cancer of the lung, cirrhosis of the liver, and cancer of the pancreas. Although the comorbidity with chronic bronchitis and emphysema was especially pronounced in patients with gastric ulcer, the association with liver cirrhosis and cancer of the pancreas occurred only in patients with duodenal ulcer. In women the mortality rate attributable to cardiac and vascular diseases was lower than expected. No excess coincidence of suicide was found. Berkson's fallacy is considered to be of much less importance as a possible explanation of the comorbidity found in the present study than in the majority of publications concerned with this question.