PURPOSE: To identify factors associated with the development of hydrops and affecting its clinical outcome. METHODS: Chart review of all patients with acute hydrops seen by a referral cornea service during a 2.5-year period between June 1996 and December 1998. RESULTS: Twenty-one patients (22 eyes) with acute hydrops were seen. Nineteen patients had keratoconus, 2 had pellucid marginal degeneration, and 1 had keratoglobus. Twenty-one of 22 (95%) eyes had seasonal allergies and 20 of 22 (91%) eyes had allergy-associated eye-rubbing behavior. Six of 22 (27%) had a diagnosis of Down's syndrome. Six patients were able to identify a traumatic inciting event: vigorous eye rubbing in 4 and traumatic contact lens insertion in 2. The affected area ranged from 7% to 100% of the corneal surface area and was related to disease duration and final visual acuity. Proximity of the area of edema to the corneal limbus ranged from 0 to 2.3 mm and was also related to prognosis. Three serious complications were observed: a leak, an infectious keratitis, and an infectious keratitis and coincidental neovascular glaucoma. Various medical therapies did not differ significantly in their effect on outcome, and ultimately 4 (18%) of 22 patients underwent penetrating keratoplasty. Best-corrected visual acuity was equal to or better than prehydrops visual acuity in 5 of the 6 patients in whom prehydrops visual acuity was known, without corneal transplantation. CONCLUSIONS: Allergy and eye-rubbing appear to be important risk factors in the development of hydrops. Visual results are acceptable in some patients without surgery. Close observation allows for the early detection and treatment of complications such as perforation and infection.