The etiology of male breast cancer has been difficult to investigate, given its relative rarity with an occurrence less than 1% that of female breast cancer. Most previous studies have involved case-control designs, raising questions about the possibility of recall bias. Analyses of individual prospective studies to identify risk factors are difficult given typically small numbers of accrued cases. In an attempt to overcome these limitations and increase statistical power, NCI investigators initiated the Male Breast Cancer Pooling Project. They have secured international collaborations resulting in the pooling of 21 studies which have provided more than 2,400 cases of this rare malignancy and 52,000 controls. They are currently exploring a variety of potential risk factors through assessment of questionnaire data and biologic material.