Cyr61 is a secreted pro-angiogenic factor that belongs to an emerging family of growth regulators classified as CCN (CTGF/Cyr61/NOV). Work in our laboratory has focused on sex steroid regulation of Cyr61 and its role in hormonal carcinogenesis. In this study, both Cyr61 mRNA and protein were induced by the progestin, R5020, in T47D mammary adenocarcinoma cells in a dose- and time-dependent fashion. Cyr61 gene induction by R5020 was transcriptionally regulated by progesterone receptor (PR) as the antiprogestin, RU486, and actinomycin D blocked induction completely. Moreover, Cyr61 was upregulated by epidermal growth factor (EGF) but not by R5020 in the PR-MDA-MB-431 mammary adenocarcinoma cell line, underscoring the necessity of PR. The functional significance of progestin induction of Cyr61 in breast cancer cell growth was demonstrated by anti-Cyr61 neutralizing antibodies, which diminished R5020 and EGF-dependent DNA synthesis by 30%. Moreover, anti-Cyr61 neutralizing antibodies reduced the synergistic effects of R5020 and EGF on T47D cell growth by 30%. Accordingly, protein lysates generated from stage II invasive ductal carcinomas (n = 20) were analyzed in order to determine the relevance of Cyr61 expression in the context of breast tumorigenesis. Remarkably, increased Cyr61 protein expression was observed in greater than 50% of primary breast tumor lysates that were progesterone receptor (PR)+ but estrogen receptor negative. Taken together, our data suggest that in addition to its proangiogenic activity, Cyr61 may be a novel mediator of progesterone activity in enhancing growth-factor-driven tumor growth in breast cancer.