The pathogenesis of thyroid-associated ophthalmopathy is thought to be autoimmune, although environmental and genetic factors are also considered to be important. As the morbidity of thyroid-associated ophthalmopathy is considerable and treatment often unsatisfactory, there a need to identify possible predisposing factors. The study was undertaken in order to ascertain the relationship between age, gender and severity of thyroid-associated ophthalmopathy. One hundred and one consecutive patients with thyroid-associated ophthalmopathy who presented over a period of 5 years to a combined thyroid-eye clinic. Patients were assessed by grading their inflammatory signs and degree of diplopia, and by measurement of exophthalmos, palpebral aperture, differential intraocular pressure, and visual acuity. On the basis of the above an ophthalmopathy index was devised to grade the overall severity of eye disease. The mean age was 49.2 years (SD 13.4), the female-to-male ratio was 4.05, and mean ophthalmopathy index 6.49 (SD 3.3). Optic nerve compression was present in 9.9% of patients. There was a positive relationship between age and ophthalmopathy index (P < 0.001); after correcting for age, males had an average ophthalmopathy index 41% greater than that of females. There is an association between severity of thyroid-associated ophthalmopathy and (i) advancing age and (ii) female-to-male ratio, which has not been previously described. Patients over the age of 60 (particularly males) with Graves' disease appear to be at risk of developing severe eye disease.