['The assessment of microsatellite instability (MSI) is not included yet in the routine evaluation of patients with gastric cancer, as controversial data exist regarding its prognostic value.', 'We determined the clinical significance of MSI in 510 sporadic gastric cancers, using the mononucleotide markers BAT25 and BAT26. The results were compared with the immunohistochemical expression of the mismatch repair proteins Mlh1 and Msh2.', 'MSI was present in 83 (16%) cancers and correlated with better survival (P < .001). Multivariate analysis showed that the MSI phenotype was an independent factor (P = .005) and added prognostic information to TNM stage, location, and age. The relative risk of death for MSI cancer patients was 0.6 (95% confidence interval [CI], 0.4-0.8). Moreover, when grouped according to stage, only stage II cancers showed a significant effect of MSI status on survival (P = .011; hazard ratio = 0.3; 95% CI, 0.1-0.8). MSI also correlated with older age (P = .002), female gender (P < .001), intestinal histotype (P = .011), lower T stage (P = .018), and less lymph node involvement (P < .001). Finally, comparison of the results of immunohistochemical expression of the mismatch repair proteins Mlh1 and Msh2 with microsatellite analysis showed concordant results in 95% of neoplasms, with a sensitivity of 82% and specificity of 98%.'] Is microsatellite instability in gastric cancer associated with better prognosis in only stage II cancers?