Mach bands, a visual phenomenon resulting from lateral inhibitory impulsess in the retina, are recognized aslucent or dense lines at the borders of different radiographic densities. A number of clinical situations havebeen described in which Mach bands may cause difficulty in radiographic diagnosis. Photodensitometric measurementof the film can differentiate the true change in film density from the Mach band which is an optical illusion.Authors present several examples of photodensitometric tracings of Mach bands, with the brief review of themechanism of their production.