Objective To compare the clinical effectiveness and cost benefit of different frequencies of scale and polish (S&P) treatments in combination with different types of oral hygiene advice (OHA).Design Multi-centre, multi-level cluster randomised factorial open trial with blinded outcome evaluation. UK dental practices were cluster randomised to deliver OHA as usual or personalised. In a separate randomisation, patients were allocated to receive S&P 6-monthly, 12-monthly or never.Setting UK primary dental care.Participants Practices providing NHS care and adults who had received regular dental check-ups.Main outcome measures The percent of sites with bleeding on probing, patient confidence in self-care, incremental net benefits (INB) over three years.Results Sixty-three practices and 1,877 adult patients were randomised and 1,327 analysed (clinical outcome). There was no statistically significant or clinically important difference in gingival bleeding between the three S&P groups (for example, six-monthly versus none: difference 0.87% sites, 95% CI: 1.6 to 3.3, p = 0.48) or between personalised or usual OHA groups (difference -2.5% sites, -95%CI: -8.3 to 3.3, p = 0.39), or oral hygiene self-efficacy (cognitive impact) between either group (for example, six-monthly versus none: difference -0.028, 95% CI -0.119 to 0.063, p = 0.543). The general population place a high value on, and are willing to pay for, S&P services. However, from a dental health perspective, none of the interventions were cost-effective.Conclusion Results suggest S&P treatments and delivering brief personalised OHA provide no clinical benefit and are therefore an inefficient approach to improving dental health (38% of sites were bleeding whatever intervention was received). However, the general population value both interventions.