The tortuosity of retinal blood vessels is an important diagnostic indicator for a number of retinal pathologies. We applied robust quantitative tortuosity metrics, which are well suited to automated detection and measurement, to retinal fluorescein images of normal and diseased vessels exhibiting background diabetic retinopathy, retinitis pigmentosa and retinal vasculitis. We established the validity of the mean tortuosity (M) and the normalized root-mean-square tortuosity (K) by their strong correlation with the ranking of tortuosity by an expert panel of ophthalmologists. The low prevalences of the diseased conditions in the general population affect the classification process, and preclude the use of tortuosity for screening for all of these conditions simultaneously in the general population. Tortuosity may be useful as a screening test for retinitis alone, and may be useful for distinguishing diabetic retinopathy or vasculitis from normal in a discretionary (i.e. referred) population.