Bladder tumors from Egyptian patients with a high prevalence of bilharziasis were immunohistochemically analyzed for the expression of cytokeratin 20 (CK20), AN43, prostaglandin dehydrogenase (PGDH), and cyclooxygenase-2 (COX-2). The tumors included 26 transitional cell carcinomas (TCC), 10 squamous cell carcinomas (SCC) and 2 tumors of mixed TCC/SCC histology. Of the 28 TCC tumors, 21 (75%) expressed CK20 and 25 (89%) expressed AN43. CK20 was not expressed in any of the 10 SCC tumors and AN43 was expressed in 2 of them. PGDH was expressed in 18 (64%) of the 28 tumors with TCC histology and 1 of the 10 SCC. A subset of 21 tumors (16 TCC and 5 SCC) was tested for COX-2 expression. COX-2 was detected in 69% of the 16 TCC tumors examined but was not seen in the SCC tumors. As tumors increased in stage, the expression of these proteins changed. CK20, AN43 and PGDH decreased but COX-2 expression increased in higher stage tumors. The histologic phenotype of these cancers is reflected in their expression of these proteins and is modified further as tumors progress in stage.