Adult neuroblastoma (ANB) is a rare and poorly recognized entity among a histologically defined group of small, round-cell tumors arising in the retroperitoneum and abdomen. Eight cases of ANB were compared with seven cases of primitive neuroectodermal tumor (PNET) in these locations to identify clinicopathologic features that could be used to distinguish between the two lesions. The ANB study group included four men and four women 22-74 years of age (mean 38 years). Five patients with ANB presented with inflammatory symptoms or elevated levels of catecholamines and their metabolites. Five of the ANB tumors were classified as undifferentiated and three as poorly differentiated with a background of neuropil. These cases often showed immunoreactivity for multiple neural markers such as CD56, chromogranin A, synaptophysin, neurofilament, and neuron-specific enolase, but were negative for CD99, cytokeratins, desmin, myogenin, smooth muscle actin, muscle-specific actin, CD34, S-100 protein, and CD45. In contrast, all of the PNETs were positive for CD99, and four (57%) were also positive for cytokeratins. Two cases of ANB of the undifferentiated subtype had ultrastructural features characteristic of neuroblastoma and lacked a chimeric transcript (EWS-FLI1or ERG), which is specific for PNET. All five patients with the undifferentiated subtype of ANB and six of the seven patients with PNET died of their disease within 3 years of discovery of the lesion. Our results show that ANB, although rare, should be considered in the differential diagnosis of patients with small, round-cell tumors in the retroperitoneum and abdomen. Appropriate immunohistochemical studies and laboratory examination enable pathologists to distinguish ANB from other differential diagnoses, especially PNET.