Background: Purpose of this retrospective study was to analyze outcome and complications after penetrating keratoplasty (PK) in patients with Down's syndrome. Patients and methods: The medical records of all patients with Down's syndrome and PK from 1985 were reviewed retrospectively. Age, sex, systemic diseases, communication, indication for surgery and surgical technique, age at first PK, complications, visual acuity and best corrected visual acuity and findings at last follow-up were noted. Age matched mentally non-retarded patients with keratoconus and PK served as a control group. Results: All eyes of 9 patients had keratoconus, 16 were operated. Indication for surgery was spontaneous perforation in 3 and refractive in the remaining 13 eyes. Five grafts were clear at last follow-up. In 7 eyes, at least re-PK had to be performed. Seven eyes developed graft rejection and 4 eyes needed conjunctival grafting because of non-healing epithelial defects. Ten eyes developed cataracts, 3 secondary glaucoma. In the control group 22 out of 25 operated corneas were clear at last follow-up. Conclusions: Prognosis of PK in patients with Down's syndrome is worse than in other patients with keratoconus. Poor results are not only explained due to the lack of cooperation of patients, but also because of: 1) high rate of emergency procedures (3/16), 2) presence of lid anomalies and 3) very advanced keratoconus, necessitating bigger sized grafts, leading to frequent immunological reactions, suture loosening and secondary glaucoma. Indication for PK in Down's syndrome patients with keratoconus and floppy eyelids should be very restrictive. Postoperative complications and the high rejection rate might be decreased by performing deep lamellar keratoplasty as this would not be an intraocular procedure.