We studied 31 eyes with nonischemic temporal branch retinal vein obstruction (TBRVO) of six months' duration or less. Each eye had an initial and at least one follow-up fluorescein angiogram according to the clinical course. Five eyes had sufficient additional capillary nonperfusion by angiography to warrant reclassification into the ischemic category. This conversion to an ischemic TBRVO pattern was documented as early as one month and as late as 14 months (average 5.2 months) after initial evaluation. Males dominated the progressive group (P less than 0.005); age, sex, initial visual acuity, and associated medical and ocular conditions were not significantly different between progressive and nonprogressive eyes (P greater than 0.05). A nonischemic TBRVO may convert to an ischemic pattern, placing these eyes at increased risk for the subsequent development of neovascular complications.