Reduced epithelial adhesion in cat corneas after continuous wear of thick hydrogel contact lenses has been reported previously. To investigate the mechanism(s) underlying this observed loss of epithelial adhesion further, the corneas of both eyes of cats that had worn low-oxygen-transmissible thick parallel-design hydrogel contact lenses only in one eye for 8-121 days were examined using both light and transmission electron microscopy (TEM). Contact lens wear induced many changes in the epithelium, including a decrease in the number of cell layers and appearance of cuboidal rather than columnar basal cell shapes. In addition, TEM revealed that the number of hemidesmosomes (HDs) per micrometer of basement membrane was reduced significantly after contact lens wear. Anchoring fibrils in lens-wearing corneas appeared normal, and the reduction in epithelial adhesion occurred without obvious epithelial edema. Decreased epithelial adhesion after contact lens wear appears to be directly related to the reduced numbers of HDs. Possible reasons for decreased HD density, such as loss of basal cell shape and chronic epithelial hypoxia after contact lens wear, are discussed.