We studied immunohistochemical expression of MUC1, MUC2, and MUC5AC in 194 carcinomas of different primary sites to determine whether differential expression patterns could be used to distinguish different carcinomas. MUC1 was expressed by most (except adrenocortical and hepatocellular carcinomas). MUC2 was expressed infrequently (positive immunoreactivity primarily in tumors of gastrointestinal origin). MUC5AC was expressed by most pancreatic ductal and endocervical adenocarcinomas and a variable number of tumors of the gastrointestinal tract. A MUC1+/MUC2-/MUC5AC- immunophenotype was observed in most breast, lung, kidney, bladder, endometrial, and ovarian carcinomas; MUC1+/MUC2-/MUC5AC+ was characteristic of pancreatic ductal adenocarcinomas and cholangiocarcinomas. Adrenocortical and hepatocellular carcinomas were negative for all mucins. Carcinomas of gastrointestinal origin exhibited variable expression of each mucin examined and no consistent immunoreactivity pattern. Many carcinomas can exhibit distinct MUC1, MUC2, and MUC5AC expression patterns, which might be valuable diagnostically in specific settings (eg, distinguishing cholangiocarcinoma from hepatocellular carcinoma or renal from adrenocortical carcinoma). However the overlapping and heterogeneous patterns of MUC1, MUC2, and MUC5AC expression observed in many tumors, particularly those of gastrointestinal origin, preclude use of these markers in the routine immunohistochemical assessment of carcinomas of an unknown primary site.