The aetiology of ophthalmoplegia in 15 patients with carotid-cavernous sinus fistula is discussed, and the clinical findings are correlated with angiographic and orbital CT appearances. After closure of the fistula the majority of patients with generalised ophthalmoplegia recovered full ocular movements rapidly, while patients with an isolated abduction weakness required much longer to return to normal. Orbital CT studies showed enlarged extraocular muscles in the patients with generalised ophthalmoplegia but muscles of normal size in those with abduction failure alone. After closure of the fistula repeat CT studies of patients with enlarged extraocular muscles showed a diminution in muscle size. We suggest that generalised ophthalmoplegia in carotid cavernous sinus fistula is due to hypoxic, congested extraocular muscles. Isolated abduction weakness is due to a sixth nerve palsy, which probably occurs either in the cavernous sinus or more posteriorly near the inferior petrosal sinus. A combination of these 2 mechanisms may be found in some patients.