Von Hippel-Lindau (VHL) disease is a dominantly inherited familial cancer syndrome caused by germline mutations in the VHL tumor-suppressor gene. Central nervous system (CNS) and retinal hemangioblastomas are highly vascular tumors that are hallmarks of the disease. These tumors overexpress vascular endothelial growth factor (VEGF) and represent a potential target for anti-angiogenic drugs. We observed, after 3 to 4 months of treatment, secondary paradoxical polycythemia in 3 VHL patients with CNS or retinal hemangioblastomas treated by the anti-VEGF receptor SU5416. Hematocrit was normal before the beginning of the trial, and no progression of hemangioblastomas was observed. Polycythemia vera and all known causes of secondary polycythemia were also ruled out. Polycythemia has never been reported in current SU5416 trials for advanced malignancies and could express a specific action on red blood cell precursors occurring only in the absence of a functional VHL gene. These findings could also affect the inclusion of VHL patients with pre-existing polycythemia in future anti-VEGF receptor trials.