Abstract: A hand-held diagnostic unit simplifies the process of monitoring the teeth for the presence of plaque, or other foreign matter, in the oral cavity. The unit includes a source of filtered light and a mirror, both contained within the same housing. The mirror is inclined relative to the front vertical wall of the housing, and a spring-loaded switch on the housing activates the light. The user coats the teeth with a substance which fluoresces when illuminated with light of the proper frequency, and which therefore enhances the visibility of plaque. The user holds the diagnostic unit opposite the teeth, activates the light, and points the unit so as to direct light towards the area of interest, while monitoring the teeth by looking at the mirror of the diagnostic unit. The inclination of the mirror makes it easy to view most areas of the mouth and to determine whether plaque is present. The diagnostic unit can therefore be operated with one hand, and can be easily transported from one location to another.