The involvement of prolactin in human tumourogenesis has been long debated. The reason is that the evidence supporting the role of circulating prolactin in promoting breast cancer was mainly obtained using rodent models, whereas most of the studies performed in human species in the 1980s have remained inconclusive. Things have now started to change because two alternative mechanisms of prolactin actions in tumour growth have emerged since the beginning of the 21st Century. The first involves locally-produced prolactin, which acts by an autocrine/paracrine mechanism. Genetically-modified mouse models have demonstrated the tumourigenic potential of local prolactin on the prostate and the mammary gland, and arguments are now emerging in humans also. The second mechanism involves genetic variants of the receptor. Although no genetic disorder has been reported for prolactin or its receptor, a variant of the prolactin receptor exhibiting constitutive activity has been recently identified in patients presenting with breast tumours, suggesting that sustained prolactin signalling may participate in breast tumourogenesis. Recent data regarding these two nonclassical mechanisms of prolactin action are discussed. Finally, we address the question of their inhibition in future cancer therapy, both in light of other findings that have revealed novel actions of prolactin in breast cancer cells, and with respect to the compounds currently available to target prolactin receptor signalling.