Previously, a modest association was observed between the progesterone receptor +331G/A gene variant and breast cancer risk. Here, in a larger sample of breast cancer cases and controls (n = 1,322/n = 1,953) nested in the Nurses' Health Study cohort, we confirm a significant association (odds ratio, 1.41; 95% confidence interval, 1.10-1.79) and suggest a molecular model. The association of the +331G/A variant with breast cancer was particularly strong among obese women (body mass index > 30; odds ratio, 2.87; 95% confidence interval, 1.40-5.90). To help understand the molecular mechanism by which this variant may predispose women to breast cancer, we identified nearby transcription factor binding sites. This search predicted a binding site for the GATA family of transcriptional regulators adjacent to this hPR polymorphism. Importantly, we found GATA3, GATA4, and GATA6 are expressed in normal breast tissue and two breast cancer cell lines, whereas GATA5 is minimally expressed in normal mammary tissue and more strongly expressed in two breast cancer cell lines. This finding was relevant because GATA5 bound the site adjacent to the +331G/A polymorphism, and activated the hPR (-711 to +822)-luciferase reporter plasmid in breast cancer cells. Overexpression of GATA5 increased expression of the endogenous hPR transcript, and GATA5 more strongly activated an hPR promoter construct encoding the PR-B isoform. Finally, hPR promoter constructs including the +331A were more strongly activated by GATA5 than constructs including +331G. Our findings suggest that GATA5 interacts with the +331G/A polymorphism to stimulate hPR-B expression in mammary cells, which may contribute to breast cancer susceptibility.