A choroid plexus carcinoma from the lateral ventricle of a ten-month-old boy was examined by light and electron microscopy. The neoplasm exhibited papillary and solid areas, invasion of adjacent brain, cytologic anaplasia, and necrosis. Intra- and extracellular PAS positive droplets were present, but no pigment was observed. No glial fibrillary acidic protein was detected within neoplastic cells. Ultrastructural examination revealed irregular nuclei with large nucleoli and occasional intranuclear filamentous rods. Cytoplasmic organelles included a moderate number of free ribosomes, rough endoplasmic reticulum, Golgi complexes, and microtubules. In some areas, numerous mitochondria filled the cytoplasm. Surface specializations included zonulae adherentes, microvilli, and clusters of cilia with a 9 + 0 microtubular pattern. Clusters of basal bodies with prominent basal feet and striated rootlets were present within the cytoplasm. Normal choroid plexus from this patient also had 9 + 0 cilia. In contrast, choroid plexus papillomas from two other infants had cilia with a 9 + 2 pattern. These observations demonstrate that both differentiated and anaplastic choroid plexus neoplasms have the potential to form cilia, and that the microtubular structure of such cilia may deviate from the usual 9 + 2 pattern.