There is no unanimity of opinion in the literature on the existence of a, possibly dose-related, mammary tumourigenic effect of progestagens in mammals. The present report comprises three studies. In the first it was established that long-term low-dose administration of a progestational compound (lynestrenol) protected beagle dogs against mammary tumours, whereas high doses were associated with an increased risk of mammary tumours, including carcinomas. In the second, a case-control field study in dogs, it was observed that administration of low doses of progestagens to prevent estrus resulted in a slightly increased risk of benign mammary tumours but not of carcinomas. The third study, a case-control field study in cats, revealed that progestagens used for estrus prevention or treatment of dermatological problems considerably increased the risk of mammary carcinoma if given regularly, but not if given irregularly. The results of these three studies indicate a dose-related tumourigenic effect of progestagens for the development of mammary tumours in dogs and cats. Similar results have been reported for rats and non-human primates. These observations in animals may require further studies on the risk of progestagen use in man and on the mechanism of the tumourigenic action of progestagens.