The value of the fibrotic focus (FF) as a marker of intra-tumoral hypoxia in invasive breast carcinoma was assessed by studying its relationship with the expression of the hypoxia-induced carbonic anhydrase IX (CA IX), angiogenesis indices and prognosis. CA IX expression was immunohistochemically detected in 2 independent study groups, totaling 184 patients, and correlated with tumor characteristics, angiogenesis related parameters and patient outcome by univariate analysis. CA IX immunostaining scores in carcinoma cells and in tumoral fibroblasts were significantly higher in expansively growing tumors (p = 0.0001 and p < 10(-4), respectively), containing an FF (p = 0.0004 and p < 10(-4)) and showing high histological grade (p = 0.016 and p = 0.0006). Microvessel density, quantified by Chalkley counting, was correlated with CA IX expression both in the carcinoma cells and in the fibroblasts (p = 0.0076 and p = 0.0025) and with the presence and relative size of an FF (p = 0.006). The fraction of proliferating endothelial cells was positively correlated with CA IX scores in the fibroblasts (r = 0.4, p = 0.02) and with the presence of an FF (p = 0.02). CA IX scores in the fibroblasts--and to a lesser extent in the carcinoma cells--were associated with a higher relapse rate (p = 0.006) and a worse overall survival (p = 0.003). The highest CA IX immunostaining scores were found in the fibroblasts of large FF occupying more than one-third of the tumor. A large FF was associated with worse overall survival in a consecutive patient group (p = 0.01) and with shorter disease-free (p = 0.02) and overall survival (p = 0.0005) in T1-2N0 breast cancer patients. The strong association of CA IX expression with the presence of an FF shows that the latter is a marker of intra-tumoral hypoxia. FF is useful as a surrogate marker of hypoxia-driven ongoing angiogenesis and is associated with a higher relapse rate and a worse overall survival.