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Background: Gallbladder cancer is very common in Chile and is the leading cause of cancer deaths in women aged over 40 years. However, there is limited information about the molecular changes involved in its pathogenesis. Microsatellite analysis was performed using polymerase chain reaction (PCR)-based assays to identify genetic loci that were altered in neoplastic and preneoplastic conditions of early and advanced gallbladder cancer. Our findings were then correlated with clinicopathological variables and survival time., Methods: We selected 59 surgical specimens of gallbladder adenocarcinomas (29 early cancers and 30 advanced cancers) and 22 surgical specimens from patients with chronic cholecystitis from a high-risk area for gallbladder cancer (Temuco, Chile). Laser capture microdissection (LCM) was used to harvest tumor cells and preneoplastic lesions. Microsatellite analysis was performed using 13 different markers. The tumors and preneoplastic lesions were also examined with immunohistochemistry for hMLH1, hMSH2, and hMSH6., Results: We found that 10% (6/59) of gallbladder cancers showed high-frequency microsatellite instability (MSI-H), with identical proportions in both early and advanced cancers. In premalignant lesions adjacent to the six MSI-H tumors, we detected instability in two of six examples of intestinal metaplasia (33%) and five of six examples of dysplasia (83%). All MSI-H cases showed an altered pattern with the antibodies studied. MSI status was not associated with survival or other clinicopathological features. No MSI or immunohistochemical alterations were found in the chronic cholecystitis group., Conclusions: Microsatellite instability was observed in equal proportions in early and late cancers, and it was also found in premalignant lesions, indicating that inactivation of mismatch repair genes occurs early in gallbladder carcinogenesis.