The Early Treatment Diabetic Retinopathy Study included use of nonsimultaneous stereoscopic fluorescein angiography to assess severity of characteristics such as capillary loss and fluorescein leakage and to guide treatment of macular edema. Two 30 degrees photographic fields were taken, extending along the horizontal meridian from about 25 degrees nasal to the disc to about 20 degrees temporal to the macula, and a classification system was constructed to allow assessment of selected characteristics. This classification system relies on comparisons with standard and example photographs to evaluate the presence and severity of capillary loss and dilatation, arteriolar abnormalities, leakage of fluorescein dye (including characterization of source), abnormalities of the retinal pigment epithelium, cystoid changes, and several other features. The classification is described and illustrated and its reproducibility between graders assessed by calculating percentages of agreement and kappa statistics for duplicate gradings of baseline angiograms. Agreement was substantial (weighted kappa, 0.61 to 0.80) for severity of fluorescein leakage and cystoid spaces, and moderate (weighted kappa, 0.41 to 0.60) for capillary loss, capillary dilatation, narrowing/pruning of arteriolar side branches, staining of arteriolar walls, and source of fluorescein leakage (microaneurysms versus diffusely leaking capillaries).