Neuroblastoma is a unique tumor that occurs during childhood. Tumors in infants less than one year of age often regress spontaneously and usually have an excellent prognosis, whereas those in older patients are aggressive, leading to a fatal outcome. To shed light on these unique aspects of this tumor, we investigated the clonality of neuroblastomas by analyzing the inactivation patterns through methylation of the human androgen receptor gene on the X chromosomes in female patients. Neuroblastoma tissue samples were obtained from 12 patients, including 10 patients less than one year of age with tumors at stage 1, 2 or 4S and 2 older patients with stage 4 tumors. Except for 3 uninformative samples of infants, all the informative samples unexpectedly exhibited random methylation pattern, and tumor tissues purified with microdissection technique exhibited the same results. These data indicate that neuroblastomas examined are polyclonal in origin, an unusual finding for a neoplasm, which might be relevant to the unique aspects of neuroblastoma.