Defects in the structure or function of the primary cilium, an antennae-like structure whose functional integrity has been linked to the suppression of uncontrolled kidney epithelial cell proliferation, are a common feature of genetic disorders characterized by kidney cysts^,. However, the mechanisms by which primary cilia are maintained remain poorly defined. von Hippel-Lindau (VHL) disease is characterized by the development of premalignant renal cysts and arises because of functional inactivation of the VHL tumour suppressor gene product, pVHL^,. Here, we show that pVHL and glycogen synthase kinase (GSK)3β are key components of an interlinked signalling pathway that maintains the primary cilium. Although inactivation of either pVHL or GSK3β alone did not affect cilia maintenance, their combined inactivation leads to loss of cilia. In VHL patients, GSK3β is subjected to inhibitory phosphorylation in renal cysts, but not in early VHL mutant lesions, and these cysts exhibit reduced frequencies of primary cilia. We propose that pVHL and GSK3β function together in a ciliary-maintenance signalling network, disruption of which enhances the vulnerability of cells to lose their cilia, thereby promoting cyst formation.