Breast cancer in a young person is considered a rare and very aggressive disease. The theories addressing the underlying genetic mechanisms of this disease are controversial. Therefore, additional genetic concepts playing a possible role in its pathogenesis and prognosis must be investigated. Microsatellite instability (MSI) characterized by a mutational process of insertions or deletions in microsatellite repeats might constitute a sensitive indicator for genomic instability in cancer. MSI has been described in a wide variety of tumors, particularly in hereditary non-polyposis colorectal cancer. The reports regarding its occurrence and prognostic significance in breast cancer are in conflict with each other. The purpose of this study was to investigate MSI in early-onset breast cancer and to correlate its occurrence with clinicopathological prognisticators. In this study, 16 female patients with primary breast cancer under 35 years of age (range 29-34) were investigated for the incidence of MSI in five microsatellite loci (D2S123, D3S1611, D17S807, D17S796 and Xq11-12) by comparing paired normal and tumor tissue DNA after PCR amplification from paraffin-embedded tissues. No instability was found in any of these five microsatellite loci. Although care must be taken not to overstate the importance of this result due to the inadequate number of microsatellite markers and DNA samples studied, this preliminary report indicates that MSI phenotype is uncommon in human early-onset breast cancer. Therefore, it does not appear to be related to the prognosis of disease.