In breast cancer, axillary lymph node invasiveness is the major prognostic factor in predicting relapse and metastasis. Nevertheless, since 30% of node-negative tumors also relapse, it is necessary to develop other independent prognostic factors. Oncogene amplification and the level of cathepsin D (cath-D), an acidic lysosomal protease produced and secreted in excess by breast cancer cells, have been proposed as additional prognostic factors. We have compared the cytosolic cath-D level and the amplification of three oncogenes: c-myc, neu-erb-B-2 and int-2 in 140 primary breast carcinomas and 64 axillary lymph nodes collected in 1987 and 1988 at the Cancer Center of Montpellier (Centre Paul Lamarque). None of the patients had previously received hormonal or chemotherapy. The cath-D concentration was measured with an immunoradiometric assay using monoclonal antibodies. DNA purified from the same samples was analyzed by a standard Southern blotting technique to estimate oncogene amplification. No correlation was found between the level of cath-D in the tumor and node invasiveness. Using a cut-off level of 60 pmol/mg protein, the status of cath-D was not correlated with neu-erb-B-2 and int-2 amplification and only correlated with c-myc amplification (P = 0.011). Both c-myc and cath-D are associated with cell proliferation, induced by estrogens in ER+ breast cancer, and constitutively produced in ER- breast cancer. The level of cath-D was significantly higher in the invaded lymph nodes (P = 0.04) than in the histologically non-invaded ones. Nevertheless, some non-invaded lymph nodes contained a high level of cath-D, as confirmed by immunoperoxidase staining. In conclusion, in breast cancer, a high cytosolic cath-D concentration is more frequent in tumors with c-myc amplification but is dissociated from neu-erb-B-2 or int-2 amplification, suggesting that the determination of these three markers will have an additional prognostic value.