Introduction It is 25 years since Lancaster1 coined the word "aniseikonia" to describe unequal ocular images which Ames and his colleagues at Dartmouth had investigated as a result of researches initially concerned with basic studies in physiological optics. Although many careful studies5-23 have revealed relief of symptoms from wearing aniseikonic corrections, especially asthenopia and certain types of headache aggravated by close work, in only a comparatively small percentage of refractions is aniseikonia considered as a concomitant factor. Because it is our belief that correction of aniseikonia should be a part of the service provided patients, we thought it important to call attention to the current literature on aniseikonia and to point out some simple, inexpensive, though approximate, procedures to be followed in the measurement and correction of aniseikonia. Donders2 (1864) was, no doubt, the first to discuss the difference in retinal image size due to the spectacle