Autosomal dominant polycystic kidney disease (ADPKD) largely results from mutations in the PKD1 gene leading to hyperproliferation of renal tubular epithelial cells and consequent cyst formation. Rodent models of PKD suggest that the multifunctional hormone insulin-like growth factor-1 (IGF-1) could play a pathogenic role in renal cyst formation. In order to test this possibility, conditionally immortalized renal epithelial cells were prepared from normal individuals and from ADPKD patients with known germline mutations in PKD1. All patient cell lines had a decreased or absence of polycystin-1 but not polycystin-2. These cells had an increased sensitivity to IGF-1 and to cyclic AMP, which required phosphatidylinositol-3 (PI3)-kinase and the mitogen-activated protein kinase, extracellular signal-regulated protein kinase (ERK) for enhanced growth. Inhibition of Ras or Raf abolished the stimulated cell proliferation. Our results suggest that haploinsufficiency of polycystin-1 lowers the activation threshold of the Ras/Raf signalling system leading to growth factor-induced hyperproliferation. Inhibition of Ras or Raf activity may be a therapeutic option for decreasing tubular cell proliferation in ADPKD.