The kinetics of cytotoxic T lymphocytes (CTL) in a patient with stage IV-S neuroblastoma were examined for 2 months. CTL were detectable during the preoperative and postoperative periods in this stage IV-S neuroblastoma patient, in spite of the tumor-bearing condition. Furthermore, the patient's mother showed the same level of CTL. This experiment suggests that patients with stage IV-S neuroblastoma have a constantly high level of CTL, and that this immunologic response is predominantly hereditary.