Corneal sensitivity and thickness were measured in nine subjects. Two different types of hard contact lenses were fitted to each subject; a traditional polymethylmethacrylate (PMMA) lens on one eye and a gas-permeable cellulose acetate butyrate (CAB) lens on the other eye. Corneal sensitivity did not differ significantly between the two eyes after 6 and 10 hr of wear, but PMMA lenses tended to induce slightly more corneal edema than CAB lenses. As oxygen tension behind such contact lenses is different, whereas mechanical stimulation is nearly the same, this would appear to support the view that corneal sensitivity changes are related to mechanical stimulation. However, careful scrutiny of the data does not support this view unequivocally, and other suggestions are made.