During a monitoring study related to photorefractive keratectomy, objective tests with the Oqual (a device that can be attached to the slit-lamp) were made of the image forming quality of treated, central, and untreated peripheral regions of the cornea. In a significant number of cases the untreated part was optically inferior to the treated part. Observations on 183 patients implicated prior contact lens wear: patients who had worn hard or gas permeable lenses scored less well than those wearing soft ones or none at all. The effects of age and length of wear were analysed. With one exception, corneal quality did not correlate with age or with length of contact lens wear either for the total sample (A) or for those aged 40 years and less (B). In A, all contact lens wearers scored less well than those who had never worn any. In B, whose eye lenses were most probably more transparent than those of the older group (A-B), those who had worn hard or gas permeable lenses scored significantly less than soft contact lens wearers or those who had never worn any. Although visual acuity is unimpaired, the optical capacity of the affected peripheral corneal regions appears to be permanently degraded, and the observation may have a potential bearing--for example, on the choice of contact lens types selected for cosmetic reasons.