The molecular mechanism of gastric tumourigenesis has not yet been clarified, although investigators have postulated that differentiated adenocarcinoma may arise from pre-existing adenoma, similarly to the colorectal adenoma-carcinoma sequence. An allelotype analysis has been performed to identify chromosomal regions which are frequently deleted in gastric tumours and to examine the significance of the adenoma-carcinoma sequence in gastric tumourigenesis. Forty-five gastric tumours, 20 adenomas, and 25 differentiated adenocarcinomas were examined for loss of heterozygosity (LOH) using 39 microsatellite markers covering each non-acrocentric chromosome arm. Frequent LOH in the adenocarcinomas was observed on chromosomes 2q (33 per cent), 4p (33 per cent), 5q (50 per cent), 6p (33 per cent), 7q (43 per cent), l l q (36 per cent), 14q (38 per cent), 17p (45 per cent), 18q (36 per cent), and 21q (40 per cent). In contrast, the incidence of LOH in adenomas did not exceed 10 per cent at any of the loci examined. In addition to the p53 gene on 17p and the DCC gene on 18q, which are known to be frequently deleted in differentiated adenocarcinomas of the stomach, other unknown tumour suppressor genes on the above-mentioned chromosomes may also be inactivated. These observations suggest that the adenoma-carcinoma sequence is not a major pathway in gastric tumourigenesis.