Atypical teratoid/rhabdoid tumor (AT/RT) is a new entity among malignant pediatric brain tumors. This study was performed to investigate the clinicopathologic and cytogenetic features of the tumor. Five cases from a series of the brain tumors were studied. Clinical features of the patients were assessed with age, sex, location of the tumors, treatments and survival periods. Histopathologic features were analyzed by routine HE stain and immunohistochemical stains for epithelial membrane antigen, cytokeratin, vimentin, smooth muscle actin, desmin, GFAP, S-100 protein, neurofilament protein, synaptophysin and alpha-feto protein. Cytogenetic studies for karyotype analysis and fluorescent in situ hybridization were available in three cases. Mean age of patients was 5.6 years, and maximal survival periods were less than 13 months despite surgical and radiation therapy. The tumors were located in infratentorial and supratentorial areas. Histopathologically, the tumors were chiefly composed of rhabdoid cells, modified rhabdoid cells and undifferentiated small cells, mixed with epithelial, mesenchymal, and neural tumor-like areas. These polyphenotypic features of the tumor cells were supported by diverse immunoreaction. Monosomy of chromosome 22 was demonstrated in two cases of the tumor. These results suggest that AT/RT may be a unique clinicopathologic entity, and histopathologic diagnosis of it should be made judiciously by differentiating other polymorphous tumors of the brain.