Using immunohistochemistry or Western blot analysis, the authors have studied the expression of the Hu antigen (a neuronal protein identified by the serum of patients with small cell lung cancer and paraneoplastic encephalomyelitis/sensory neuronopathy) in normal human tissues and 115 tumors of different histologic types. In normal tissue, the Hu antigen is highly restricted to the nervous system. In lung tumors, the Hu antigen is restricted in its expression to all small cell carcinomas. A few other neuroendocrine-related tumors, especially neuroblastomas (50%), also express the antigen.