The Multicenter Trial of Cryotherapy for Retinopathy of Prematurity (CRYO-ROP) was a randomized trial designed to test whether cryotherapy applied to the retina significantly decreased the incidence of retinal detachment, retinal fold, or retrolental "mass." Randomization into the trial was terminated on recommendation of the Data and Safety Monitoring Committee 9 months before the scheduled end of the intake period. At the time of the decision, a favorable outcome at 3 months posttreatment occurred more frequently in eyes treated with cryotherapy (78%) as compared to untreated eyes (57%) (chi 2 = 20.5). The basic statistical method used in monitoring the results was conditional power (or stochastic curtailment). Several other issues were considered in deciding to terminate the trial, including the examination of results across subgroups and clinical centers, the completeness of follow-up, clinical assessment of outcome, and long-term consequences of the treatment.