Mammary tumors in ruminants The frequency of mammary neoplasia in different species varies tremendously. Among women, breast cancer features as one of the most frequent types of cancer globally. Breast cancer is the cause of almost half a million deaths worldwide each year. Of these deaths, more than 58 % are occurring in developing countries. Mammary tumors are rare in cows, mares, goats, ewes, and sows as evident by few cases reported in literature. In dairy cattle and milking goats the udders are inspected daily by palpation when milking and the tumors would be detected soon. The morphological features of bovine mammary gland are more similar to those in human than in rodents, the latter being commonly used for cancerogenic studies. There are several factors that might affect the high rate of tumor development in women and attenuate the development of tumors in ruminants. Besides diet, life style, reproductive history, stress load, the general physiology and metabolism might have an influence. Translational research using different mammary gland cell populations from several species may lead to comparative studies and may help reduce the risk and contribute to discovery of new therapeutic targets to treat breast cancer. In present work the zero occurrence of mammary gland tumors in ruminants will be reviewed focusing on literature records and mechanisms underlying the resistance.