The pre- and post-operative ophthalmic consequences of fronto-ethmoidal mucocoeles are considered in a prospective series of 22 patients. Proptosis, lateral and inferior displacement, diplopia and restriction of ocular mobility were the commonest presenting problems. Following external fronto-ethmoidectomy, proptosis and displacement resolved completely in the majority of patients. One third of patients experienced a transient exacerbation of diplopia but this resolved in most patients. Persistent diplopia occurred in seven patients and required medical correction in four and surgery in one. This was due in all cases to underaction of the superior oblique muscle and probably to backward displacement of the trochlea which may be avoided by careful surgical re-attachment.