Based on the anatomical and clinical findings in the very rare cases reported, recent experimental data, and the results of oculography, the authors consider that this condition merits definition as an individual entity. It is usually associated with other disorders of oculomotility which may mask clinical signs, and oculography is necessary to establish the diagnosis. Terminologically, it would appear logical to retain the terms; abduction internuclear ophthalmoplegia (and not, posterior), and adduction internuclear ophthalmoplegia (and not, anterior).