Recent studies suggest that corneal acidosis may alter corneal structure and function. We determined whether the range of oxygen transmissibility (Dk/L) provided by a hydrogel lens could affect corneal stromal pH. Stromal pH was measured using a noninvasive fluorometric technique under both open- and closed-eye conditions on subjects who wore hydrogel lenses made of the same material but with different lens powers. Under closed-eye lens wearing conditions, central stromal pH was reduced substantially and there was no relation between degree of acidosis and lens Dk/L. Under openeye lens wearing conditions, central stromal pH was also reduced but the degree of acidosis was dependent on lens Dk/L. These results suggest extended wear of hydrogel lenses can produce marked decreases in stromal pH which could remain reduced even after the eyes are opened.