The incidence of gastric carcinoma was studied in a series of 210 patients presenting with apparently benign gastric ulcer. In eight cases (3.9 per cent), carcinoma was diagnosed within 18 months and was almost certainly present from the outset; in 2.4 per cent the diagnosis was delayed for over three months, and the five-year death-rate due to gastric carcinoma was 3.2 per cent. In a mean follow-up period of 5.7 years after the first diagnosis of an ulcer (8.2 years after first symptoms), three fresh cases of gastric carcinoma were found, and in two of these the cancer was at a different site from the ulcer. The five-year incidence of fresh gastric carcinoma was 0.6 per cent. Unsuspected superficial spreading carcinoma was detected by histology in 5.4 per cent of gastrectomy specimens. The significance of these findings is discussed in relation to the management of gastric ulcer and the early diagnosis of cancer.