A 1.85-kg, 6-yr-old, captive-bred, male boa constrictor (Boa constrictor imperator) was presented for lethargy, anorexia, postural abnormalities, and had an elongated mass on its ventrum, 20 cm distal to the snout. Clinical examination revealed a firm, nonmobile coelomic mass (4 cm × 2 cm) and loss of the righting reflex. Hematology showed a significant increase in white blood cells, lymphocytosis, and anemia. Cytologic examination of the blood smears showed the presence of lymphoid leukemia and eosinophilic intracytoplasmic inclusions consistent with inclusion body disease (IBD). Hyperphosphatemia was suggestive of renal failure. Radiography and ultrasound revealed a soft tissue mass at the level of the thymus proximal to, and distinct from, the heart. Cytology and postmortem histopathology confirmed the presence of a multicentric lymphoblastic lymphoma, lymphoid leukemia, and IBD. It remains unclear whether the neoplasms began their proliferation within the bone marrow or whether leukemia was a feature of disseminated, end-stage lymphoma.