AIM: To investigate the association between loss of heterozygosity (LOH) on chromosome 18 and sporadic gastric cancer.METHODS: Multiplex PCR was used to screen 14 highly polymorphic microsatellite markers on chromosome 18 in45 cases of primary gastric cancer. PCR products were separated on polyacrylamide gels and the electrophoresis maps were analyzed with Genescan and Genotyper.RESULTS: The LOH frequencies in gastric cancer at all 14markers ranged from 10% to 58%. Eleven markers were found with over 20% LOH frequencies, in which 9 markers located in 18q, and 2 markers in 18p. Two overlapping deleted regions were identified: R1 between D18S61-D18S1161 at 18q22 (9cM) with 24% LOH frequency; R2 between D18S462-D18S70 at 18q22-23(6cM) with 32% LOH frequency.CONCLUSION: LOH of chromosome 18 (18q and 18p) may be involved in gastric tumorigenesis. Two overlapping deleted fragments suggested that there might be unidentified tumor suppressor genes in those two regions.