• A short-term study of the physiologic response of the cornea to polymethyl methacrylate (PMMA) contact lenses and hard, gas-permeable contact lenses of different materials (polycon and Menicon O2), thicknesses, and fittings, was carried out on nine unadapted subjects using one eye as the experimental eye and the other as the control eye (no lens). Changes in corneal thickness were monitored at seven locations across the cornea using an electronic micropachymeter. It was found that gas-permeable lenses produced less corneal swelling than the PMMA lenses. However, lens thickness and the fit of the lens were still important control variables with these gas-permeable lenses. The thin gas-permeable lens produced less swelling than thicker lenses, and the flatter fitting gas-permeable lenses produced less swelling than the steeper fitting lenses. The swelling with all the hard lenses studied was confined primarily to the central 6 mm of the cornea.