Background. Mooren's ulcer is a rapidly progressive, painful, ulcerative keratitis which initially affects the peripheral cornea and may spread circumferentially and then centrally. Mooren's ulcer can only be diagnosed in the absence of an infectious or systemic cause and must be differentiated from other corneal abnormalities, such as Terrien's degeneration. Although the etiology remains unknown, recent research has proposed an underlying immune process and a possible association with the hepatitis C virus. The response to medical and surgical intervention is typically poor, and the visual outcome can be devastating. Case Series. Three patients presented to our clinic with inferior peripheral corneal defects characteristic of Mooren's ulceration. The first patient, a 67-year-old white male, presented with an area of progressive peripheral thinning of the left inferior cornea 1 week after a preoperative skin cleanser was inadvertently splashed in both eyes. This occurred during a surgical procedure to remove a basal cell carcinoma. The second patient, a 56-year-old white male, was treated for a recurrent left inferior corneal ulcer with impending risk of perforation. The third patient was a 68-year-old white male referred for a painful left inferior peripheral ulcer, which rapidly progressed into a bilateral corneal melt disorder. All patients were diagnosed with Mooren's ulcerative keratitis after they underwent extensive medical and laboratory testing to rule out an infectious or systemic cause of their corneal melt. The first patient was treated with oral steroids, as well as doxycycline, to control his acne rosacea. The second patient responded to aggressive treatment with topical steroid therapy. This patient also tested positive for hepatitis C. The third patient rapidly developed a perforated left cornea and was treated with a penetrating keratoplasty after a patch graft had failed. Discussion. Mooren's ulcer is an idiopathic, painful, progressive ulceration of the peripheral cornea. These ulcers usually respond poorly to conventional therapy, as there is limited knowledge of the pathophysiology of the disease. Evidence of an autoimmune component advocates the use of steroids and immunosuppressive agents. With further research and understanding of Mooren's ulcer, better treatment options may be available in the future.