Histidine decarboxylase is one of the enzymes of the amine precursor uptake and decarboxylation system and is known to be distributed in mast cells and enterochromaffin-like cells. With the hypothesis that histidine decarboxylase expression is a marker for neuroendocrine differentiation, we studied the immunoreactivity of histidine decarboxylase in neuroendocrine cells and tumors of the thyroid gland, adrenal medulla, lung, and gastrointestinal tract. Formalin-fixed paraffin sections were subjected to immunohistochemistry using anti-histidine decarboxylase antibody, and the sensitivity and specificity were compared with those of conventional neuroendocrine markers (CD56, chromogranin A, synaptophysin, and neuron-specific enolase). Enterochromaffin or enterochromaffin-like cells, adrenal chromaffin cells, and thyroid C-cells were positive for histidine decarboxylase, and related tumors (carcinoid tumor, pheochromocytoma, medullary carcinoma) showed a high percentage of positive staining. Furthermore, we used the antibody to distinguish small cell lung carcinoma from non-neuroendocrine lung carcinoma and also to detect neuroendocrine differentiation in large-cell neuroendocrine carcinoma and gastrointestinal small-cell carcinoma. The anti-histidine decarboxylase antibody stained most small cell lung carcinoma (18 of 23, sensitivity 0.78), and was rarely reactive with non-neuroendocrine lung tumors (2 of 44; specificity, 0.95). These values were close to those obtained from CD56 staining (sensitivity/specificity, 0.87/0.98). Histidine decarboxylase was also positive for 6 of 12 large cell neuroendocrine carcinomas and 4 of 7 gastrointestinal small cell carcinomas. In conclusion, we demonstrated that histidine decarboxylase is useful to distinguish between small cell lung carcinoma and non-neuroendocrine carcinoma and to demonstrate neuroendocrine differentiation.