A 13-year-old male green tree monitor (Varanus prasinus) presented with lethargy and chronic weight loss. Complete blood count (CBC) revealed marked leukocytosis (total leukocyte count = 278.1 × 10³/μl) and peripheral blood films were characterized by marked lymphocytosis comprised of small, well-differentiated lymphocytes. Bone marrow aspiration revealed increased cellularity with a predominance of well-differentiated lymphocytes. Using immunocytochemistry, neoplastic lymphocytes in peripheral blood films were positive for CD3 epsilon and negative for CD79a and BLA.36, consistent with T-cell chronic lymphocytic leukemia (CLL). Distal tail necrosis was noted 17 days post-presentation and treated with partial caudectomy. Treatment was initiated for CLL with 0.8 mg/kg prednisone q 48 h 22 days post-presentation. CBC on days 94 to 151 post-presentation showed marked reduction in leukocytosis (counts remaining below 40.0 × 10³/μL). The animal displayed improvement in activity, appetite, and gained weight during the initial treatment period. Marked leukocytosis (253.4 × 10³/μL) returned on day 203 post-presentation, and on day 206 post-presentation the monitor received 0.08 mg/kg chlorambucil PO once in addition to ongoing prednisone administration. The animal died 210 days post-presentation. Histopathologic examination revealed neoplastic lymphocyte infiltration in the bone marrow at multiple sites as well as in liver, spleen, gall bladder, pancreas, kidneys, esophagus, and the oral cavity. Lymphocyte infiltration in multiple organs was primarily associated with areas of dense lymphocytic infiltrates surrounding blood vessels. This report describes the first published attempt of CLL treatment in a saurian species.