Notch3 has been studied in the context of brain development, but whether it plays a role in the formation of brain tumors is unclear. We demonstrate that the introduction of constitutively active Notch3 into periventricular cells of embryonic day 9.5 mice causes the formation of choroid plexus tumors (CPTs). Tumors arose in the fourth ventricles in 83% of animals and were associated with hydrocephalus. They were microscopically highly similar to choroid plexus papillomas in humans, with an ongoing proliferation rate of 4-6%. Signs of Notch pathway activity were also present in human choroid plexus lesions, and receptor mRNA levels in papillomas were elevated over those in non-neoplastic choroid plexus. Notch2 was overexpressed approximately 500-fold in one case, suggesting that the role of this pathway in CPTs may not be specific to Notch3. Our findings indicate that activated Notch3 can function as an oncogene in the developing brain, and link the Notch pathway to human CPT pathogenesis.