Metastatic progression in ductal breast carcinomas are related to apoptosis in primary tumors. Frameshift mutations in a single-repeat sequence within the coding region (G)8 of the pro-apoptotic Bax gene have been related to microsatellite instability (MSI) and progression of some carcinomas and lymphomas. The aim of this study was to explore whether the extended lifespan of breast cancer cells can also be triggered by Bax mutation in ductal-breast carcinomas, and whether breast cancer cell MSI is related to the loss of apoptosis. For this purpose we studied frameshift mutations of a microsatellite (G)8 in the third exon of the Bax gene in a series of 105 ductal breast carcinomas, at T1 and T2-3 stages, 45 of which had lymph node metastasis. We analyzed MSI in five sequences of DNA isolated from normal and tumor tissue samples taken from 86 patients, and we explored the relationship between MSI and tumor apoptosis status. Bax mutation was not present in ductal breast carcinomas. MSI (two or more markers altered) was detected in 11.6% of tumors. Loss of apoptosis occurred in 80% (8/10) tumors with MSI, versus 17.8% of tumors without MSI (chi2 test, p = 0.0004), independently of Bax protein expression. We conclude that frameshift mutations of a microsatellite (G)8 of the Bax gene are not critical for the loss of apoptosis in breast cancer, and that loss of apoptosis may be a consequence of overexpression of anti-apoptotic protein Bcl-2 or Bcl-xL. Moreover, MSI in breast carcinomas might be the cause of loss of an apoptotic pathway that is not induced by frameshift mutations of a microsatellite (G)8 of the Bax gene.