BackgroundFilamentary keratitis is an ocular condition that is tricky to handle for the difficulty to find the underlying cause. Here we report a case of filamentary keratitis associated with Demodex infestation which highlights the importance of Demodex mites as an easily-overlooked risk factor.Case presentationA 63-year-old woman had recurrent symptoms of foreign body sensation and sometimes painful feelings in her left eye soon after her surgical correction of ptosis in this eye. She was then diagnosed as conjunctivitis and given antibiotic eye drops. After one week, the patient complained of aggravation of symptoms with small corneal filaments in the left eye under slit-lamp examination. Despite the removal of filaments and addition of topical corticosteroids and bandage contact lenses, the patient’s condition persisted with enlarged filaments and severe ocular discomfort. 3 days later, eyelashes with cylindrical dandruff were noticed and Demodex infestation was confirmed by microscopic examination of these eyelashes at our clinic this time. She was asked to use tea tree oil lid scrub twice daily. After 3 weeks, her filamentary keratitis was resolved with a dramatic improvement in symptoms and signs. And no recurrence of filamentary keratitis was noticed during the one-year follow-up.ConclusionsIn this case, filamentary keratitis was resolved only with treatment of Demodex infestation while conventional treatment failed. Considering the fact that Demodex infestation is a common but easily overlooked condition, it may be suggestive to take Demodex infestation into account as a risk factor of filamentary keratitis, especially in refractory cases.