Endoscopy is a valuable tool in the diagnosis and management of duodenal lesions and biliary strictures. We assessed the value of cytology in the evaluation of these lesions and analyzed the causes of discrepancy among clinical, histologic, and cytologic parameters. The study included 118 patients with duodenal ulcers, ampullary neoplasms, or biliary strictures who were examined between 1975 and 1995; 120 cytologic examinations were performed. The specimens included brushings of the duodenum (DB, n = 50), ampulla (AB, n = 32), and biliary ducts (BB, n = 38). Endoscopic biopsies performed concurrently included the duodenum (n = 37), the ampulla (n = 22), and the biliary ducts (n = 23). Comparison of cytologic and histologic results showed the following sensitivity and specificity: DB, 40% and 97%, respectively; AB, 100% each; BB, 75% and 93%, respectively. The DB, AB, and BB revealed malignant neoplasms in 2 of 5, 7 of 7, and 6 of 8 cases, respectively. Twenty-three duodenal neoplasms were diagnosed by either modality and included 11 adenocarcinomas, 9 villous tumors, 2 metastatic renal cell carcinomas, and 1 large cell non-Hodgkin's lymphoma. Endoscopic brush cytology is an effective means of diagnosing ampullary neoplasms, and it complements tissue biopsy in cases of bile duct stricture. Location, predominance of tumor-induced stroma, an extramucosal growth pattern, sampling error, and interpretative experience influence the diagnostic evaluation. Cytologic diagnosis of an adenoma does not exclude an underlying malignant neoplasm in ampullary tumors. In some instances, it may be difficult to distinguish between villous tumors with severe dysplasia and adenocarcinomas by cytology alone.