The primary outcome measure of the effectiveness of cryotherapy in the original design of the Multicenter Trial of Cryotherapy for Retinopathy of Prematurity (CRYO-ROP) was the anatomic status of the retina, as documented by fundus photography 3 months and 12 months after infants had received treatment. The authors describe the addition of a measurement of visual function, the Acuity Card procedure, to the CRYO-ROP study. After training, four visual-acuity testers attempted to measure monocular grating acuity in all randomized infants and approximately one fourth of the natural-history infants in the study, tested at 1 year postcryotherapy or 1 year postterm. In 95% of infants on whom testing was attempted, monocular acuity values from each eye were obtained. Interobserver test-retest results on 25 eyes of 13 randomized infants agreed to within one octave or better in all but one of the eyes. The high testability rate and good interobserver agreement suggest that the Acuity Card procedure has been a successful method of assessing visual function in the CRYO-ROP study.