Objective: Histological evidence is considered the only proof of primary central nervous system vasculitis (PCNSV). However, brain biopsy is often omitted or delayed because of the invasiveness and possible complications of the procedure. Circulating endothelial cells (CEC) were shown to be elevated in patients with active antineutrophil cytoplasmic antibody-associated vasculitis. We hypothesise that CEC are also elevated in patients with active PCNSV and may contribute to the diagnosis. Methods: CEC were assessed in 18 patients, 3 of whom had biopsy-proven PCNSV and 15 clinical, cerebrospinal fluid and imaging data, highly suggestive of PCNSV. In 3 of these 15 patients CEC assessment was performed after initiation of successful immunosuppressive therapy. CEC numbers of all patients were compared to those of 16 healthy volunteers and 123 subjects with cerebrovascular risk factors and/or ischaemic stroke, who had been studied in our group before. CEC were assessed by immunomagnetic isolation from peripheral blood. Results: In patients with proven and suspected active PCNSV, CEC were extremely elevated (>400 cells/ml in most of the patients) and signifi cantly higher than in healthy and disease controls (p≤0.01 for each group). CEC significantly decreased with immunosuppressive treatment. Conclusions: For the first time it is shown that CEC are significantly elevated in patients with active PCNSV in contrast to other pathologies associated with brain infarction and correlate with disease activity. Sensitivity and specificity of the method for diagnosing PCNSV and the use of the method for treatment monitoring should be addressed in future prospective studies with a larger patient group.