We studied the lysosomal enzyme activities in corneas obtained from 12 patients with keratoconus. Three acid hydrolases--acid phosphatase, acid esterase, and acid lipase--were demonstrated by histochemical staining methods in the epithelium, stroma, and endothelium of corneas with keratoconus and normal human corneas. Analyses by an image-processing system indicated that the epithelium, especially the basal epithelium, of corneas with keratoconus had significantly higher levels of acid phosphatase, acid esterase, and acid lipase than those in normal human controls. Such an abnormality was not seen in either scarred corneas or corneas obtained from patients with Fuchs' corneal dystrophy. Our finding is consistent with the hypothesis that tissue degradation processes may be abnormal in keratoconus. In addition, our data suggest that the corneal epithelium, as previously speculated, may also be involved in this corneal disease.