Tumor-associated monocytic cells (TAMs) are a major component of the stroma responsible for tumor formation. TAMs generate various kinds of mediators for their function, one of which is thymidine phosphorylase (TP). TP is an angiogenic enzyme that is known to be up-regulated in tumor tissues. Here, we focused on the clinical implication of TP expression in TAMs by studying 229 primary breast carcinoma tissues. Immunohistochemical analysis demonstrated that monocytic TP+ tumors had a significantly worse prognosis than did monocytic TP- tumors (P < 0.01, log-rank test). A multivariate analysis confirmed that monocytic TP status provided an independent prognostic value (P < 0.0001). Furthermore, of interest was that monocytic TP status could categorize the CD68+ patients, who had an extensive accumulation of CD68+ TAMs, into two subgroups with strikingly contrasting prognoses: a good prognostic monocytic TP- group and a poor prognostic monocytic TP+ group. This indicates that there are both antitumor and protumor types of TAM. Subanalysis showed that microvessel density was significantly increased in CD68+/monocytic TP+ tumors compared with CD68+/monocytic TP- tumors. Experimentally, TAMs are known to function in diverse manners, antitumor and protumor; however, little is known about clinically recognizable markers to characterize the TAMs in histological sections. TP might be such a marker, which would be useful for identifying the character of TAMs, particularly the protumor phenotype.