A 24-year-old, female cockatiel (Nymphicus hollandicus) was diagnosed and treated for chronic lymphocytic leukemia. Diagnosis was based on a persistent lymphocytosis, with counts increasing 10 times from reference intervals with a high percentage of well-differentiated lymphocytes in the bone marrow. Immunohistochemical staining confirmed neoplastic cells of T-cell origin. Specific treatment for the disease was initially withheld but commenced based on an increasing lymphocytosis and decreasing packed cell volume. Therapeutic management of the cockatiel's chronic lymphocytic leukemia was based on human protocols. Treatment with chlorambucil stabilized the disease but did not result in a significant regression of the neoplasm. The bird was euthanatized 15 months after the initial diagnosis and 8 months after treatment commenced. On the postmortem examination, extensive T-cell lymphocytic infiltration was found throughout the internal organs.