Aim The study aimed to test the effectiveness and acceptability of a smartphone application used in conjunction with a movement sensor toothbrushing attachment for promoting plaque control. Method One hundred and eight dental practice patients were recruited to the study from two general dental practices. Participants were randomised to test and control groups, and both groups offered oral hygiene instruction according to a single protocol. Test participants were given the smartphone device and toothbrush attachment. Control patients were not. After two and four weeks, full mouth plaque scores of the mouths of both test and control participants were measured. A comprehensive questionnaire administered to the test group assessed participants' views about the acceptability of the smartphone device and application. Results Full mouth plaque scores declined from 40.1 to 11.7, a reduction of 70% in the test group compared to a reduction from 29.1 to 20.5 (30%) in the control group. The device was found to be very well accepted. Participants were conscious of improving their brushing and improving their knowledge of how to brush well. They also reported enjoyment and fun being derived from use of the device and found it simple to use. Conclusion Providing immediate day-to-day feedback to dental patients about their brushing results in dramatic improvements in oral hygiene and highly significant reductions in plaque levels, in at least the short-term; beyond that seen in previous toothbrushing interventions with adult patients.