OBJECTIVE: To determine the spectrum of clinical presentations of hyperthyroidism in cats and response to carbimazole therapy by analysis of historical, clinical and laboratory data. DESIGN: A prospective clinical study involving client-owned cats presenting to a private veterinary practice in Australia. PROCEDURE: Twenty-five cats diagnosed as hyperthyroid during a 23-month period participated in the study with owner consent. Therapy with carbimazole was instituted and revisits were scheduled 2, 6 and 13 weeks after diagnosis. The cats were physically examined and underwent haematological and serum biochemical testing at each revisit. Owners were also asked to assess clinical signs in their cats in the periods between veterinary examinations. Cats with underlying renal disease were managed by alterations or cessation of carbimazole therapy. RESULTS: A high prevalence of lethargic or inappetent cats without detectable underlying nonthyroidal illness was found. There was also a high prevalence of cats less than 10-years-old and cats in good body condition. Fourteen cats treated with carbimazole and monitored for 13 weeks responded favourably to therapy. Side-effects were minor and uncommon. Cats with underlying renal disease that became apparent during the study, responded well to alterations or cessation of carbimazole therapy. CONCLUSION: The trend towards more subtle clinical presentations of hyperthyroid cats reported previously continued in this study. The findings of the current study do not appear to support the traditional view of hyperthyroid cats as being old, hyperactive, hungry and thin. Carbimazole therapy was found to decrease the prevalence of almost all clinical abnormalities in 14 cats and side-effects were minor and uncommon. This study demonstrates the usefulness of medical management of hyperthyroidism in the cat when radioiodine therapy is not possible due to renal compromise or other factors.