CA 19-9 is a tumor marker of pancreatic and gastrointestinal cancer. Elevation in nonmalignant disease is rare. The case of a patient with a partial staghorn calculus, giant hydronephrosis, and elevated CA 19-9 serum levels is presented. Open transperitoneal right-sided nephrectomy was performed. In immunohistochemical analysis, CA 19-9 was expressed in the renal tubular epithelium and the renal pelvis. During postoperative follow-up, the CA 19-9 levels returned to normal. Hydronephrosis might cause false-positive results when CA 19-9 measurement is used to screen for malignant disease. Posttreatment CA 19-9 levels of patients with hydronephrosis have to be monitored closely to safely exclude malignant disease.