Non-compliance with oral self-care recommendations, despite education and motivation, is a major problem in preventive dentistry. Forming concrete if-then action plans has been successful in changing self-care behavior in other areas of preventive medicine. This is the first trial to test the effects of a brief planning intervention on interdental hygiene behavior. Two hundred thirty-nine participants received a packet of floss, information, and a flossing guide. They were randomly assigned to a control or an intervention group. The intervention took 1.16 minutes and consisted of forming a concrete plan of where, when, and how to floss. Baseline measures and two-week and two-month follow-ups included self-report, residual floss, and theory of planned behavior variables. The intervention significantly affected flossing in that group at two-week and two-month follow-ups, as compared with the control group. This study provides evidence for the effects of a concise intervention on oral self-care behavior.