In autosomal dominant polycystic kidney disease (ADPKD), aberrant proliferation of the renal epithelial cells is responsible for the formation of numerable fluid-filled cysts, massively enlarged kidneys, and progressive loss of renal function. cAMP agonists, including arginine vasopressin, accelerate cyst epithelial cell proliferation through protein kinase A activation of the B-Raf/MEK/extracellular signal-regulated kinase (ERK) signaling pathway. The mitogenic effect of cAMP is equally potent and additive to growth factor stimulation. Here, we determined whether Sorafenib (BAY 43–9006), a small molecule Raf inhibitor, inhibits proliferation of cells derived from the cysts of human ADPKD kidneys. We found that nanomolar concentrations of Sorafenib reduced the basal activity of ERK, inhibited cAMP-dependent activation of B-Raf and MEK/ERK signaling, and caused a concentration-dependent inhibition of cell proliferation induced by cAMP, epidermal growth factor, or the combination of the two agonists. Sorafenib completely blocked in vitro cyst growth of human ADPKD cystic cells cultured within a three-dimensional collagen gel. These data demonstrate that cAMP-dependent proliferation of human ADPKD cyst epithelial cells is blocked by Sorafenib and suggest that small molecule B-Raf inhibitors may be a therapeutic option to reduce the mitogenic effects of cAMP on cyst expansion.