This review will focus on the role for prolactin (PRL) and growth hormone (GH) in mammary tumor formation. Much attention has previously been focused on circulating levels of GH/PRL in relation to mammary tumor formation. We will review data demonstrating that these ligands also could be produced locally in different organs, including the mammary gland and mammary tumors, and suggest that this local production may be of importance for pathological conditions. We will also discuss mechanisms for crosstalk between steroids and GH/PRL. A crosstalk between GH-and PRL response is possible at multiple levels. In the human, GH can activate both the prolactin receptor (PRLR) and the growth hormone receptor (GHR). We have demonstrated that activation of the PRLR, but not the GHR, is inducing mammary tumors in transgenic mice. Furthermore, the elevated levels of insulin-like growth factor 1 (IGF-I) seen in the GHR activating transgenic mice is not sufficient for tumor induction. The induced tumors express functionally active prolactin that could be of importance for the tumor formation. Paracrine/aurocrine stimulation by PRL may be more important than PRL transported via the circulation. In women, the role for stimulation of the PRLR and/or the GHR in mammary tumor formation has not been proven, although experiments from primates suggest that the PRLR could be of importance.