The c-myc proto-oncogene was analyzed in 311 cases of primary breast cancer, in 8% of which it was found to be amplified, usually at moderately increased copy number (2-5 copies). The adjacent pvt gene was co-amplified with c-myc in all tumors analyzed. C-myc amplification was significantly correlated to a high S-phase fraction and to amplification of the c-erbB-2 proto-oncogene. Weak relationships were found between c-myc amplification and the presence of lymph-node metastasis, advanced stage, DNA non-diploidy and premenopausal status, but not tumor size, estrogen receptor or progesterone receptor status, or int-2 amplification. C-myc amplification, and especially a high gene copy number (greater than 5 copies), was significantly related to early recurrence and death in breast cancer, a relationship seen in both the lymph-node-negative and node-positive subcategories. A particularly strong correlation with poor clinical outcome was seen in postmenopausal patients (p greater than 0.0005), an association which persisted in multivariate survival analysis. We conclude that the activation of c-myc is indeed associated with rapidly growing and progressive breast cancer. Gene amplification, on the other hand, is relatively infrequent and occurs mostly at low copy number, implying that tumors are heterogeneous with respect to cell clones harboring c-myc amplification. An immunohistochemical assessment would more accurately illustrate the importance of c-myc activation in human breast cancer. However, the obvious instability of the c-myc transcript and translate suggests that c-myc is not a suitable prognostic marker for routine purposes.