This study evaluates the replacement rate of glass and plastic CR-39 lenses and compares the durability of glass and plastic lenses in the Air Force environment. One thousand pairs of spectacles having one glass lens and one plastic lens were dispensed to military personnel at four Air Force bases with different climates. Replacement of plastic or glass lenses was made on the basis of subject response during triannual inspection visits. Comparative durability was assessed on the basis of hazemeter readings and visual inspection. A significantly higher plastic lens replacement rate was found. Plastic lens scratching compared to glass lens scratching increased with time.