Corneal specimens obtained during surgery from patients with macular corneal dystrophy and obtained at autopsy from control eyes were incubated in a medium containing radioactive precursors of glycoproteins and proteoglycans. Biosynthetically radiolabeled material was extracted and characterized by using molecular sieve chromatography and specific enzymes. Cells in control corneas synthesized both a chondroitin sulfate proteoglycan and a keratan sulfate proteoglycan similar to those present in monkey and bovine corneas. Cells in macular corneas synthesized a normal chondroitin sulfate proteoglycan but did not synthesize either keratan sulfate or a mature keratan sulfate proteoglycan. Instead, macular corneas synthesized a glycoprotein with unusually large oligosaccharide side chains. This glycoprotein was not detected in normal corneas and is slightly smaller than normal keratan sulfate proteoglycan. The failure to synthesize a mature keratan sulfate proteoglycan may produce corneal opacity and result in blindness. Because of evidence indicating that the corneal keratan sulfate proteoglycan is normally synthesized through a glycoprotein intermediate [Hart, G. W. & Lennarz, W. (1978) J. Biol. Chem. 253-5795-5801], macular corneal dystrophy may be a defect in glycoprotein processing.