We report a new method for quantitating retinal blood vessel width from fluorescein angiogram negatives using a computerized image analyser. Interuser and intrauser variability were 3 to 4 times lower than reported with older methods. In addition, model blood vessels were used to test the accuracy of our method under four clinically relevant conditions: variations in vessel width, fluorescein concentration, flash intensity and background fluorescence. Although computed measurements were affected by vessel width, fluorescein concentration and flash intensity, the linearity of the actual versus computed width was maintained during wide variations in all four conditions (r = 0.95, P less than 0.0001). Accuracy was best achieved at a flash intensity of 150 W/sec and a fluorescein concentration of 45 micrograms/ml. The results of this study provide a better understanding of factors affecting the apparent width of retinal blood vessels in fluorescein angiograms. This technique should be useful in rapidly obtaining accurate measurements of blood vessel width from fluorescein angiograms.