Four cases of choroid plexus papilloma (CPP) obtained at the time of surgical excision were examined by light and electron microscopy and compared with normal choroid plexus (CP) of mouse and chick. Ultrastructurally CPPs were the same as CP except in the following two points: In CCPs 1) pinocytotic vesicles of the vascular endothelium and 2) cytoplasmic filaments were more increased in number than in CP. In one case, which showed malignant changes in some parts, we found prominently increased spot desmosomes and cytoplasmic filaments as compared with other three cases, microcysts at the apical portion with other features suggesting the secretory process except for the absence of basal infoldings and myelin bodies. As for the apical tight junction fusion of the two outer-leaflets of the adjacent cytoplasmic membrane was observed as in CP. This fact suggests that there is a blood-CSF barrier also in CPPs as in CP.