The year 1917 witnessed the opening of a great debate in neuro-ophthalmology, with Tournay's description of a phenomenon thought to be universal: anisocoria in lateral gaze, due to mydriasis of the abducting pupil and a minor miosis of the fellow one. Through many publications, this phenomenon was soon assessed and confirmed to be physiologic, while the cause of the pupil reaction in lateral gaze was extensively discussed. Cases of abolition and reversal of the phenomenon were also reported in specific clinical situations, such as neurosyphilis, and its study became part of the neuro-ophthalmologic examination. Publications on the phenomenon declined after 1934, with only one publication appearing between 1934 and 1974 (1957). In 1974, the demise of the Tournay phenomenon was secured after 2 pupillographic studies showed it to be rare and marginal. This story is reviewed. Hypotheses are proposed in order to help unfold the controversy; anisocoria in lateral gaze should not be regarded as a myth, nor does it--or only rarely--correspond to a pupil reaction, or noumenon: it can usually be accounted for by a specific optical distortion (apparent without real anisocoria) and hence fits the Kantian definition of a phenomenon. An understanding of Tournay phenomenon is therefore of current relevance to the examination of the pupils to avoid misinterpretation of observed changes in the pupil.