We describe two cases of a unilateral fixed dilated pupil secondary to the ocular instillation of nebulised ipratropium bromide. In one patient there was no other neurological abnormality. The second patient was unconscious following a cardiac arrest. While a fixed dilated pupil is an alarming sign, if it is caused by ocular instillation of ipratropium bromide the condition will resolve, although it may take up to 24 hours. The differential diagnosis of a unilateral dilated pupil includes partial third nerve palsy, tonic pupil, direct trauma to the eye and pharmacological mydriasis. The diagnosis can often be determined using pilocarpine eye drops.