The incidence and correlations of undercorrection were studied for 1 year after excimer laser surgery for myopia or myuopic astigmatism.A consecutive series of 645 eyes of 440 patients were studied. Eyes were examined preoperatively and at 1, 3, 6, and 12 months after surgery. The parameters evaluated were visual acuity, refraction, and corneal clarity.Following excimer laser surgery, undercorrection of > or = -1.00 diopters gradually increased from 10% at 1 month to 40% at 12 months. Increasing degree of preoperative myopia was significantly associated with increasing occurrence of undercorrection at 3 months (chi 2 = 17.3, P < .001), 6 months (chi 2 = 53.6, P < .001), and 12 months (chi 2 = 64.8, P < .001). Undercorrection was more common in eyes that had had photorefractive keratectomy than in those that had had photoastigmatic refractive keratectomy (odds ratio, 0.40; 95% confidence interval, 0.25 to 0.60). At 1 year, a loss of 2 or more lines of best-corrected visual acuity was recorded in 38% of undercorrected patients. Loss of 2 or more lines of best-corrected visual acuity was more common in patients undercorrected by -1.00 diopter or more (odds ratio, 8.8; 95% confidence interval, 5.4 to 14.6). No relationship was seen between corneal haze and loss of best-corrected visual acuity. Undercorrection was not associated with age, gender, use of nonsteroidal anti-inflammatory drugs, bandage contact lens wear, or corneal haze. Patients with lower degrees of myopia reached a stable refraction more quickly. At 6 months, 71% were within +/- 0.5 diopter of 1-year refraction. Of 17 patients with undercorrection who were treated with topical corticosteroids, only one patient showed a permanent beneficial change.Occurrence of undercorrection is more common in patients with severe myopia and when simultaneous astigmatic corrections are undertaken.