Iris fluorescein angiography was used to perform the first prospective, controlled, masked evaluation of the effect of panretinal photocoagulation on diabetic rubeosis. Eyes with rubeosis had iris fluorescein angiograms five to seven weeks apart. Eyes either had laser treatment immediately after the first angiogram or had no laser treatment between angiograms. The rubeosis was said to have improved if all three masked readers independently selected the angiogram performed last as having the less severe rubeosis. In eyes with severe rubeosis, only two of 11 (18%) spontaneously improved whereas 11 of 15 (73%) improved following laser therapy. In eyes with mild rubeosis, one of ten improved spontaneously and one of ten improved following laser therapy. Iris fluorescein angiography documented regression of severe but not mild rubeosis following panretinal photocoagulation.