A nine-year-old, domestic shorthair cat was diagnosed with hyperthyroidism and treated with methimazole, which resulted in lethargy, inappetence and marked generalised lymphadenomegaly within two weeks of initiation of therapy. Cytology, histopathology and immunohistochemistry were suggestive of atypical lymphoid hyperplasia. Cessation of treatment resulted in resolution of all clinical signs and physical abnormalities within two days. Subsequent treatment with radioactive iodine cured this cat of its hyperthyroidism. The lymphadenomegaly did not return at any stage and the cat is currently asymptomatic. Although methimazole administration for feline hyperthyroidism has been associated with many side effects, lymphadenomegaly has, to the authors’ knowledge, not been previously reported.