Twenty-three centers collaborated in this prospectively designed study involving infants born weighing less than 1251 grams. Patients examinations began 4 to 6 weeks from birth. For entry into the study, two investigators had to agree that an eye had developed stage 3 ROP involving a threshold number of at least 5 contiguous or 8 total clock hour sectors of zone 1 or 2, and 'plus' disease to a degree specified by a standard photograph. Cryotherapy was lightly applied to the avascular zone between the ridge of ROP and the ora serrata, in an average of about 50 separate spots. Outcome was determined 3 and 12 months following randomization, by means of masked readings of fundus photographs. Preliminary results were published in the Archives of Ophthalmology, Vol. 106, pp. 471-479, 1988. In these results from 172 of the 291 study patients, 43.0% of the control eyes had an adverse outcome (defined as retinal detachment, macular fold, or retrolental mass), as compared to 21.8% adverse outcome for eyes that received cryotherapy.