We screened 30 gastric adenomas and 72 gastric adenocarcinomas for four genetic alterations (mutations of the K-ras, APC, and p53 genes and loss of heterozygosity at the DCC genetic locus) which are known to occur during colorectal tumourigenesis. We used polymerase chain reaction (PCR) single-strand conformation polymorphism analysis to detect mutations. Loss of heterozygosity (LOH) at the DCC locus was ascertained directly by performing PCR on the variable number of tandem repeats within the gene. Mutations of the K-ras gene were not detected in any gastric adenoma or carcinoma. APC mutations were detected in 20 per cent (6/30) of the adenomas but in only 1.4 per cent (1/72) of the carcinomas. In contrast, the p53 gene was frequently mutated in carcinomas (35 per cent; 25/72), but not in adenomas. LOH at the DCC locus was a frequent occurrence in carcinomas (58 per cent; 11/19 informative cases) but was infrequent in adenomas (14 per cent; 1/7). Alterations of the p53 and DCC genes occurred frequently both in differentiated and in undifferentiated gastric carcinomas. The considerable differences in the incidences of genetic alterations between gastric adenoma and carcinoma indicate that the sequential development of gastric carcinoma from adenoma is uncommon in gastric carcinogenesis.