It has been assumed, on the basis of research carried out more than 50 yr. ago and lacking adequate controls, that the magnitude of the Mueller-Lyer illusion is inversely related to the angle between the oblique lines and directly related to the length of the oblique lines of the figure (up to some maximum length). The present study investigated, under controlled conditions, the relation between the magnitude of the illusion and these two characteristics of the figure. Data confirmed earlier research and indicated no significant interaction between these two variables. Both angle and length of the oblique lines are linearly related to the magnitude of the illusion.