Aims and Background Amplification/overexpression of HER-2/neu and inactivation of p53 may be reliable parameters for the prognostic assessment of breast carcinomas. Several studies have addressed the prognostic significance of simultaneous expression of these gene abnormalities with controversial results. Methods In this study we analyzed the biopathological profile of 45 breast cancers with both HER-2/neu and p53 overexpression and compared their features with those of 45 randomly selected cases negative for these gene products. Results Tumors with HER-2/neu and p53 coexpression were found in younger patients, were more often multifocal and/or multicentric, were poorly differentiated in 55% of cases and lymph node-positive in 57%, showing a statistically significant difference compared to tumors with neither alteration (11% and 28%, respectively). Moreover, they were prevalently negative for estrogen (71% vs 22%) and progesterone receptors (78% vs 40%) and showed a higher proliferative activity. Conclusions Our data demonstrate that the coexpression of p53 and HER-2/neu is an additive effect in terms of genetic instability reflected by both morphological and biological adverse features; patients with such coexpression should be assigned to specific therapeutic and follow-up protocols.