This study aimed to identify the role of mouse fibroblast-mediated c-Jun and IGF-1 signaling in the therapeutic effect of finasteride on benign prostatic epithelial cells. BPH-1 cells, alone or with fibroblasts (c-Jun+/+ or c-Jun-/-), were implanted subcutaneously in male nude mice who were then treated with finasteride. The degrees of cell proliferation, apoptosis, and sizes of the xenografts were determined. BPH-1 cells were grown alone or co-cultured with mouse fibroblasts in the presence of finasteride and the level of IGF-1 secreted into the medium by the fibroblasts was determined. The proliferation-associated signaling pathway in BPH-1 cells was also evaluated. Fibroblasts and c-Jun promoted xenograft growth, stimulated Ki-67 expression, and inhibited BPH-1 apoptosis. Finasteride did not induce the shrinkage of xenografts in the combined-grafted groups despite repressing Ki-67 expression and inducing cell apoptosis. The addition of c-Jun-/- fibroblasts did not promote xenograft growth. In the absence of c-Jun and fibroblasts, finasteride did not alter xenograft growth, Ki-67 expression, or cell apoptosis. The in vitro results demonstrated that when BPH-1 cells were grown in monoculture, treatment with finasteride did not induce cell death and stimulated the expression of pro-proliferative signaling molecules, while in the presence of fibroblasts containing c-Jun, finasteride treatment repressed epithelial cell proliferation, the level of IGF-1 in the medium, and the activation of downstream pro-proliferative signaling pathways. Taken together, our results suggest that fibroblasts, c-Jun, and IGF-1 play key roles in mediating stromal-epithelial interactions that are required for the therapeutic effects of finasteride in benign prostate epithelial cells.