Serial immunocytologic analysis of blood for tumor cells in two patients with neuroblastoma. Tumor surveillance tests are used to determine whether malignant cells are responsive or resistant to therapeutic regimens. For patients with neuroblastoma, conventional methods of surveillance are not sensitive enough. Because tumor cells are shed into the circulation, immunocytologic analysis of blood may function as a sensitive monitoring system. In this study, five blood samples were obtained from two patients with disseminated neuroblastoma at diagnosis and during therapy. These samples were analyzed with monoclonal antibodies and immunoperoxidase staining to determine whether circulating neuroblasts were present. In both patients, the presence or absence of circulating neuroblasts yielded information that was more sensitive than that from conventional tests. The authors conclude that immunocytologic analysis of blood should be included with conventional monitoring methods for surveillance of patients with disseminated neuroblastoma. 