We report and explain unilateral pupillary dilatation following routine septoplasty and trimming of inferior turbinates. The unilateral pupillary dilatation was caused by inadvertent instillation of sympathomimetic, (xylometazoline hydrochloride) in the eye during preparation for nasal surgery. The effect was short-lived and the patient made a full recovery. Unilateral pupillary dilatation after sinonasal surgery can be alarming due to the possibility of injury to the globe and intracranial haemorrhage but can also be explained by the mydriatic effect of the sympathomimetic nasal drops and sprays used to decongest the nose. Such possibility should be borne in mind and would help to explain and reduce the anxiety and avoid unnecessary investigations.