Angiogenesis plays a critical role in tumor biology and may someday be a target for novel therapeutic interventions. To date, however, relatively few markers have been identified that can specifically distinguish between microvessels in benign versus malignant lesions. Here we report that the cationic heme-protein eosinophil peroxidase (EPO) was localized by in situ immunohistochemistry on the vascular endothelial cells and/or connective tissue stroma in 16 of 16 cases of human endometrial carcinoma and in 12 of 15 cases of ovarian carcinoma. Similar deposits of EPO were not detected in normal endometrial tissues or ovaries from five healthy subjects, in adjacent uninvolved tissues from four tumor-bearing subjects, or in any normal organs from five other subjects. These findings imply that eosinophil degranulation is a significant and previously unappreciated component of the interaction between ovarian and endometrial cancers and the host. Moreover, the abundant and highly specific nature of the EPO deposition near and within the microvessels of these cancers suggests that eosinophil degranulation is a new marker for tumor blood vessels that potentially could be exploited to treat these important types of cancers that currently lack highly effective therapies.