Rings of corneal edema were noted in one eye of each of three keratoconic patients, all wearing McGuire design rigid contact lenses of limited oxygen transmissibility (Dk/La). This finding appears to be analogous to the previously reported "central circular clouding" (CCC) with this specific appearance associated with the particulars of the keratoconic cornea-contact lens relation. Estimation of local oxygen transmissibilities suggests that a Dk/L < 3 x 10(-9) may produce this gross clinical sign underneath the lens midperiphery, whereas central Dk/L > or = 6 x 10(-9) may be sufficient to prevent its appearance.