Gastric malignancies developed in 15 patients with gastric polyps (seven men, eight women; mean age 66 +/- 8.8 years) during a 14-year follow-up of 170 patients (8.8 per cent). The polyps were hyperplastic in 90 per cent of the patients, adenomas in 8.2 per cent and leiomyomas in 1.8 per cent. During follow-up 4.7 per cent of the patients had both adenomas and hyperplastic polyps. Ten patients developed adenocarcinoma, two lymphoma, one lymphoid sarcoma, one carcinoid tumour and one severe dysplasia. The incidence of malignancy was sevenfold higher than expected (P less than 0.01). There is an associated but lower risk of malignancy in stomachs with hyperplastic polyps than with adenomas; 2 of the 15 stomachs with adenomas and 3 of the 142 hyperplastic polyps showed later malignancy (P less than 0.001). Before the diagnosis of malignancy 1.1 +/- 1.4 (0-4) gastroscopies were done and 12 +/- 19 (0-48) months had elapsed since the first examination and detection of polyps, whereas the mean number of gastroscopies and length of follow-up period in all patients with polyps was 1.9 +/- 0.9 (0-10) and 68 +/- 35 (0-156) months, respectively. Eleven patients had a stage I malignancy and four had a stage IV tumour. The survival rate of the eight patients with malignancy found at the first examination was better (6/8) than for those diagnosed during the follow-up period (3/7) (P less than 0.01; chi2 test). The results show the high risk of malignancy in stomach polyps, adenomas or hyperplasia, and stress the importance of careful and repeated examination of these patients.