Case Description: A 16-year-old female umbrella cockatoo (Cacatua alba) was referred to the University of Georgia College of Veterinary Medicine for evaluation of a 3-year seasonal history of lethargy and weight fluctuation., Clinical Findings: Abnormalities detected via clinicopathologic analyses included mild leukocytosis, heterophilia, and lymphopenia consistent with a stress leukogram. Previous fecal examinations failed to diagnose enteric parasite infestation. Results of a fecal flotation test with Sheather sugar solution revealed spirurid eggs (Spiruroidea). Coelomic radiography revealed a widened cardiohepatic waist with increased soft tissue opacity at the level of the hepatic silhouette. The caudal thoracic and abdominal air sacs bilaterally appeared compressed against the coleomic wall. The proventriculus was increased in diameter, with a proventriculus-to-keel ratio of 1.0. Coelomic ultrasonography and positive-contrast upper gastrointestinal radiography revealed severe thickening and irregularity of the proventricular wall. The animal was anesthetized for an endoscopic examination of the upper gastrointestinal tract. Intralesional nematodes were identified on histologic examination of biopsy specimens from the proventriculus., Treatment and Outcome: Effective fenbendazole treatment (15 mg/kg [6.8 mg/lb], PO, alternating between 5 days of treatment and 5 days of no treatment, which continued for 4 periods) was confirmed by repeated endoscopy and fecal examinations. The bird remained free of clinical signs 27 months after diagnosis and treatment., Clinical Relevance: Antemortem diagnosis of proventricular nematodiasis has not been reported in psittacines. Spirurid nematode eggs are shed intermittently, which may lead to false-negative results on a single routine fecal examination. In this patient, radiography, endoscopy, and histologic evaluation facilitated antemortem diagnosis. This is the first report of successful treatment of this condition in psittacines.