Measurements of O2 transmissibility are reported for hydrogel lenses of different power. Center thickness is related to O2 transmissibility; the relation is different for lenses of high dioptric power compared with lenses of low power. For high plus lenses, the O2 transmissibility is higher than expected from simple consideration of center thickness and the O2 permeability of the contact lens material. For high minus lenses, the O2 transmissibility is less than expected. For clinical application in humans power and center thickness must be considered when estimating the O2 transmissibility of high power lenses.