IMPORTANCE: Retroprosthetic membrane (RPM) formation is the most common complication associated with the Boston type 1 keratoprosthesis and has been associated with corneal melt. OBJECTIVE: To identify the histological and immunohistochemical characteristics of RPMs associated with corneal melt. DESIGN, SETTING, AND PARTICIPANTS: Observational histopathological case series at a tertiary eye care referral center among patients who underwent Boston type 1 keratoprosthesis explantation because of donor corneal melt at the Illinois Eye and Ear Infirmary between January 1, 2011, and January 1, 2012. EXPOSURES: Seven RPM specimens from 7 eyes were stained with hematoxylin-eosin, cytokeratin 7, cytokeratin AE1/3, smooth muscle actin, vimentin, and CD34. Light microscopy was used to evaluate specimens for inflammation and epithelial ingrowth. XY-karyotyping using fluorescence in situ hybridization was performed on 4 specimens with known donor-recipient sex mismatch. MAIN OUTCOMES AND MEASURES: Histological and immunohistochemical characteristics of RPMs. RESULTS: Inflammatory cells were present in 4 of 7 RPMs. In 3 of 4 sex-mismatched specimens, tissue XY-karyotyping of the RPM interphase cells was consistent with the host sex karyotype. The fourth specimen showed a mixture of recipient-type and donor-type cells. CONCLUSIONS AND RELEVANCE: Melt-associated RPMs showvariable degrees of inflammation. Most membranes seem to originate from a proliferation of host cells, but donor tissue may contribute in some cases.