This case report describes a 10-year-old castrated male Russian Blue cat who was referred to the hospital for respiratory distress and coughing. Tests for FeLV and FIV infection were negative. Pleural fluid was removed and subjected to cytological and lymphocyte clonality analysis. Thoracic radiography and ultrasonography confirmed atelectasis in some pulmonary lobes. The cat was diagnosed with stage III small- to medium-cell T cell lymphoma, and was treated with prednisolone (5 mg/head) and L-asparaginase (10000 units/m2). With this treatment, the pleural effusion gradually subsided and respiratory signs improved. However, the cat developed decreased motor function of the hindlimbs, forelimbs, and tongue, leading to the administration of 1-(2-Chloroethyl)3-cyclohexyl-1-nitrosourea (CCNU) (10mg/head or 38 mg/m2) for suspected central nervous system infiltration of lymphoma. Neurological signs dramatically improved within seven days, and CCNU (15mg/head or 54 mg/m2) was administered five more times until day 284, after which it was replaced with chlorambucil (2mg/head every four days). On day 393, weight loss and abdominal lymphadenopathy were observed, and CCNU (10mg/head) was administered again on the day 407 but was ineffective. Unfortunately, the cat eventually died on the day 466 with marked abdominal lymphadenopathy.