Hyperthyroidism is recognised not only as the commonest endocrine disease of domestic cats but as one of the most frequently diagnosed disorders in small animal practice. Prior to its first definitive diagnosis in 1979, there were few reports of pathological abnormalities in feline thyroid glands and only anecdotal reference to clinical signs that may have been caused by hyperthyroidism. Since that time, there has been a marked increase in the frequency of diagnosis of feline hyperthyroidism. It is unlikely that increased awareness and improved diagnostic capabilities alone account for such a dramatic increase in the prevalence of this disease and it has been suggested that hyperthy- roidism is truly a new disease of cats. However, this is complicated by a growing cat population, an increased longevity for cats and possibly, a greater propensity to seek veterinary advice.