Background and Purpose-The etiology of moyamoya disease (MMD) remains obscure. This study was undertaken to identify specific proteins associated with the pathogenesis of MMD. Methods-We studied cerebrospinal fluid (CSF) from 20 patients with angiographically confirmed MMD (4 boys and 16 girls; age range, 3 to 13 years; mean, 7.5 years) and 4 control patients with cerebral palsy who underwent selective dorsal rhizotomy (2 boys and 2 girls; age range, 5 to 10 years; mean, 7.3 years). CSF proteins were analyzed by 2-dimensional polyacrylamide gel electrophoresis, and protein identification was performed by matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry. The presence of specific CSF protein in patients with MMD was confirmed by Western blotting. In addition, cerebral CSF was also tested in 7 patients who had other brain diseases but no MMD (2 boys and 5 girls; age range, 1 to 12 years; mean, 6.9 years). Results-We identified 1 polypeptide spot (M r of 13 to 15 kDa and isoelectric point of 5 to 5.5) that was differentially expressed in the CSF samples of MMD patients (mean optical density intensity, 0.36Ϯ0.24; range, 0.05 to 0.92) and control spinal CSF samples (mean, 0.03Ϯ0.04; range, 0 to 0.08; Pϭ0.002). This polypeptide was identified as cellular retinoic acid-binding protein (CRABP)-I. High levels of expression of CRABP-I in the CSF from 17 MMD children were confirmed by Western blotting. Conclusions-The analysis of the CSF of MMD patients reveals high CRABP-I expression. The present study suggests that the elevation of CRABP-I in CSF may be a candidate for pathogenesis of MMD.