Tumor cells and their surrounding microenvironment produce a variety of growth factors and proteolytic enzymes to promote tumor growth and metastasis. We have recently identified a novel factor, termed com1, which is up-regulated in human breast carcinoma cells upon formation of experimental metastatic tumors and assumed to act as a growth-promoting factor in breast cancer. In attempts to explore the biological role of com1 in clinical tumor growth and metastasis, expression of com1 mRNA in primary carcinomas from 81 breast cancer patients and 27 samples of uninvolved adjacent breast tissue from these patients was compared and related to known prognostic parameters and outcome. The levels of com1 mRNA were significantly up-regulated (P < 0.0001) in the tumors compared to the normal breast tissues. Tumor expression of com1 mRNA, however, did not correlate with the mRNA levels for urokinase-type plasminogen activator (uPA), its receptor, or the type 1 inhibitor, which are factors that define a phenotype of tumor aggressiveness when elevated. And whereas the mRNA levels of uPA and the uPA receptor were elevated in tumors from the patients who subsequently had poor outcome, no correlations were observed between tumor com1 mRNA expression and prognosis or histological and biochemical characteristics of the tumors. We therefore assume that com1 may mediate some growth-promoting function early in development of the primary breast carcinoma, but not in later stages of tumorigenesis or metastasis.