Measurements were made on fundus photographs of the diameters of retinal arterioles in a series of cases with various nonvascular lesions of the anterior visual pathways, before and after the development of fundus signs of optic atrophy. A simple method was devised for reliable estimation of the diameter of the central retinal artery whenever this vessel could not be measured directly: this allowed comparisons to be made between different eyes, irrespective of individual variations in arteriolar branching patterns. Atrophy was consistently associated with a decrease in central retinal artery caliber, averaging 24% in eyes developing total optic atrophy, and 17% in eyes with hemi-atrophy. Normal controls showed a maximum side difference of 8%.