In order to clarify the expression of prohormone convertase (PC) 1/3 and PC2 in various carcinoids and non-carcinoid endocrine tumors, we performed indirect immunoperoxidase staining on total of 19 cases of carcinoids (9 cases of bronchial carcinoids, 4 cases of rectal carcinoids, 4 cases of gastric carcinoids and 2 cases of bile duct carcinoids). Our study also included 7 non-carcinoid endocrine tumors. Seventy-nine% and 26% of carcinoids highly or strongly expressed positive staining for PC1/3 and PC2, respectively. High and strong expressions (3+ or 4+) of both PC1/3 and PC2 were noted in only bronchial carcinoids. Strong expressions for only PC1/3 were noted in rectal carcinoids. Bile duct carcinoids also demonstrated higher expressions of PC1/3 than those of PC2. These results suggested that high expressions of both PC1/3 and PC2 in bronchial carcinoids might reflect their diverse and frequent peptide production. The expressions of PC1/3 mRNA and PC2 mRNA detected by in situ hybridization in the bronchial carcinoids and rectal carcinoids were correlated with immunoexpressions of both of the antigens. The granular immunoexpression pattern of PC1/3 and PC2 visualized by confocal laser scanning microscopy would suggest the site of post-translational processing in the secretory granules. Non-carcinoid endocrine tumors showed low expressions (+ or 2+) of PC1/3 and PC2, except for thyroid medullary carcinoma showing high immunoexpression of PC1/3. Other non-carcinoid endocrine tumors (parathyroid adenomas and adrenal pheochromocytomas) revealed low immunoexpressions for both PC1/3 and PC2.