Previous epidemiologic studies of the effects of soy protein on cancer risk have been limited by small variations in soy intake, inability to separate soy from other dietary variables and difficulties inherent in relating dietary intake to the development of cancer several decades later. As a result, although existing data suggest that soy protein may be protective for cancer risk, results are overall inconclusive. There is also evidence that soy products may affect risk factors for cancer, such as endogenous hormone levels. Preliminary data from our group indicate that young Adventist women who are vegetarians with high soy intake and a lower risk of breast cancer may have higher levels of an adrenal androgen, dehydroepiandrosterone sulfate. Other groups have noted that soy protein may be associated with alterations in the regulation and binding of ovarian hormones. Additional studies examining effects of soy protein on risk factors for cancer would help, not only in delineating mechanisms of cancer development, but also in designing dietary programs aimed at cancer prevention.