Toothbrushing with long horizontal strokes often causes traumatization of sound enamel and gingival tissues. Also, dental plaque on cervical and adjacent surfaces of teeth can hardly be removed because toothbrush tips cannot reach these regions adequately. A toothbrushing device which indicates overstroke was tested to determine the efficacy with which it makes subjects master brushing with short horizontal strokes following an initial period of instruction. The degree of learning measured in the testing group was superior to that of the control group immediately after the first instruction. Moreover after one week and 12 weeks, its efficacy persisted. The plaque elimination ratio, however, showed no significant difference between the testing group and the control.