c-erbB-2–positive breast carcinomas are highly aggressive tumors. In vitro data on breast cell lines showed that c-erbB-2 enhanced translational efficiency of hypoxia inducible factor-1α (HIF1α) production (Laughner et al., Mol Cell Biol 2001;21:3995–4005). We investigated the clinical correlate of this observation to assess whether c-erbB-2 expression was related to HIF1α expression, angiogenesis, and prognosis. A series of 180 breast carcinomas of known c-erbB-2 status (90 c-erbB-2–positive and 90 c-erbB-2–negative carcinomas) were stained immunohistochemically for HIF1α and CD31 endothelial cell antigen. c-erbB-2 positivity was clearly related to HIF1α protein expression and high angiogenesis. However, prognosis was decreased only in cases with simultaneous c-erbB-2 and HIF1α expression. If activation of c-erbB-2 in humans results in overexpression of HIF1α independently of conditions of hypoxia, as occur in experimental studies, this interaction may represent a main pathway conferring clinical aggressiveness to c-erbB-2–positive breast tumors.