The overexpression of N-myc gene and its protein products has been thought to be limited to cases of neuroblastoma, retinoblastoma and small cell lung carcinoma, but there is increasing evidence of its wider distribution in human tumors. This study showed that the protein of N-myc gene is associated in normal, benign and malignant human breast tissues. We found that N-myc oncoprotein is overexpressed in most breast carcinomas and that N-myc overexpression is significantly correlated with clinical stage, and histological grading of the tumors, and, more importantly with the clinical outcome of the patients. Analysis of DNA, mRNA and protein levels suggested that the high N-myc expression in breast cancer occurs without concomitant gene amplification. The finding of a direct inverse correlation between N-myc overexpression and the prognosis of patients with breast carcinoma suggests that N-myc expression may be useful as a prognostic factor in human breast cancer.