Macular corneal dystrophy is an autosomal recessive disorder in which abnormal deposits in the corneal stroma have been identified. We examined the corneal buttons of 12 patients, who had clinical features of macular dystrophy, by histochemical staining, transmission electron microscopy, and immunohistochemical techniques. All corneas exhibited positive staining with Muller Mowry's colloidal iron. Using monoclonal antibodies 1/20/5-D-4, J-10, J-19, and J-36 that recognize specific sites on the sulfated keratan sulfate molecule, we stained corneal sections by an avidin-biotin-peroxidase complex method and identified two groups of macular corneal dystrophy. One group consisting of four corneas reacted positively with all four antibodies, and the other group consisting of eight corneas did not react with any of the antibodies used. These results confirmed those recently presented by Yang et al that there may be subgroups of macular dystrophy that can be identified by immunohistochemical methods. Also, serum levels of sulfated keratan sulfate were determined in seven patients. One patient who displayed a normal level of serum keratan sulfate had positive corneal immunoreactivity. Of the six patients who lacked serum keratan sulfate, four showed negative and two had positive corneal immunostaining, suggesting at least three subgroups in the disease. An attempt was made to correlate the clinical features, histochemical-staining characteristics, and ultrastructural morphology with the immunoreactivity to keratan sulfate antibodies, but no correlations could be made.