Aldo-keto reductase (AKR) is a super gene family, consisting of fourteen families and more than 40 members overall. These proteins have been well known as metabolic enzymes of carbonyls, but recent data indicates that the members in AKR families 1 and 7 (AKR1 and AKR7) are involved in the development of some human and rodent tumors, such as in primary liver, lung, colorectal, prostate, and breast cancers. They are involved in the pathogenesis, diagnosis, and therapy of these tumors. This manuscript discusses the recent progression in AKR study in mammalian tumors, focusing on prostate and breast cancer.