Background Alzheimer disease (AD) and Parkinson disease (PD) involve tau pathology. Tau is detectable in blood, but its clearance from neuronal cells and the brain is poorly understood. Methods Tau efflux from the brain to the blood was evaluated by administering radioactively labeled and unlabeled tau intracerebroventricularly in wild-type and tau knock-out mice, respectively. Central nervous system (CNS)-derived tau in L1CAM-containing exosomes was further characterized extensively in human plasma, including by Single Molecule Array technology with 303 subjects. Results The efflux of Tau, including a fraction via CNS-derived L1CAM exosomes, was observed in mice. In human plasma, tau was explicitly identified within L1CAM exosomes. In contrast to AD patients, L1CAM exosomal tau was significantly higher in PD patients than controls, and correlated with cerebrospinal fluid tau. Conclusions Tau is readily transported from the brain to the blood. The mechanisms of CNS tau efflux are likely different between AD and PD.