Gas permeable (GP) lenses can flex on some eyes producing unpredictable clinical results. A method of measuring the flexibility of hard GP materials has been developed and shown to be repeatable. Materials in the form of flats rather than lenses were used. Differences between materials were found and in general a linear relation was shown to exist between maximum flexing and quoted oxygen permeability (r = 0.78, p < 0.05). It is recommended that flexibility be measured and reported in the data presented with all new GP polymers. The term “hard” rather than “rigid” in describing GP lenses is suggested.