In this study, we document a case of phenobarbital-induced anticonvulsant hypersensitivity syndrome (AHS), which has been rarely reported in veterinary medicine. A 2-year-old, 5.4 kg, neutered male Russian Blue cat was diagnosed with idiopathic epilepsy and started on phenobarbital treatment. Eight days after initiation of phenobarbital treatment, the cat showed tachypnea and hyperthermia. CBC and serum biochemistry were unremarkable. However, the patient showed high serum amyloid A (SAA). On abdominal ultrasonography, generalized enlargement of abdominal lymph nodes and splenic multiple hypo-echoic nodules, which were consistent with reactive lymphadenopathy were found. The cat was diagnosed with AHS, and phenobarbital was discontinued. After 10 days of cessation, the patient had normal SAA, and clinical signs were resolved.