To examine the relationship between peptic ulcer and gastric cancer, we investigated 2529 patients with peptic ulcer diagnosed from 1963 to 1975. During the follow-up period of 9-23 years, we found 38 in whom gastric cancer developed or who died of gastric cancer. Included were nine in whom gastric cancer was detected at the same site as the gastric ulcer initially diagnosed, and 22 in whom the gastric cancer was detected at another site. In the remaining seven, gastric cancer was given on the death certificate, but the details were unknown. When the data on gastric ulcer initially diagnosed were reevaluated, gastric cancer was suspected or could not be completely ruled out in seven of the nine in whom gastric cancer was detected at the same site. In the remaining two, a diagnosis of benign ulcer was made even when the initial data were reviewed. In these two, however, there was the possibility that the initially diagnosed gastric ulcer represents a phase of the "malignant cycle." The number of deaths from gastric cancer in patients with gastric ulcer was significantly low, compared with that expected and computed by the age- and sex-matched general population. These results suggest that gastric ulcers rarely become malignant.