Tumor specimens from 116 untreated patients with primary breast carcinoma at different clinical stages were analyzed for the structure and/or the expression of c-myc and c-erbB-2/neu proto-oncogenes. An amplification of the c-myc proto-oncogene (3 to greater than 50 fold) was detected only in 6% of carcinomas, with no evidence of locus rearrangement. High c-myc RNA levels detected in 45% of tumors were found significantly (p less than 0.01) correlated with lymph node involvement. Amplification (3 to greater than 30 fold) of the c-erbB-2/neu gene was observed in 20% of cancers. A 5 kb c-erbB-2/neu gene transcript was detected in the 103 cancer specimens analyzed. High levels of transcripts were observed in 36% of tumors. Overexpression did not depend only on amplification since found in 14 tumor samples with a single gene copy. The gene amplification and overexpression were found significantly associated with cancers of poor prognosis. Moreover our data show that both proto-oncogenes are overexpressed only in 12.5% of tumor samples and suggest that each gene might play a different role in tumor progression.