 Despite currently available screening techniques, only 63 of breast cancer cases are diagnosed at a localized stage, for which the 5-year survival rate exceeds 95 1. Advances in breast imaging are occurring, but further efforts to detect breast cancer at an early stage would be beneficial. Recently, new technologies which use mass spectrometric analysis to characterize the overall pattern of peptide expression in the serum or plasma have been developed. Preliminary studies have suggested that peptide profiles can be used to differentiate cancer patients from those without cancer for a variety of malignancies including breast cancer. If mass spectrometric analysis is able to detect differences in peptide profiles between early stage breast cancers and normal controls, this technique could be developed as a screening modality. 