ObjectiveTo examine the oral health conditions and oral health behaviour of high-cost patients and evaluate oral health measures as predictors of future high-cost patients.DesignA retrospective, population-based cohort study using administrative healthcare records.SettingThe National Health Insurance Service (NHIS) medical check-up database (a.k.a. NHIS—national health screening cohort database) in South Korea.Participants131 549 individuals who received biennial health check-ups including dental check-ups in 2011 or 2012, aged 49–88.Primary outcome measuresCurrent and subsequent year high-cost patient status.ResultsHigh-cost patients, on average, incur higher dental costs, suffer more from periodontal disease, brush their teeth less and use secondary oral hygiene products less. Some of the self-reported oral health behaviours and oral symptom variables show statistically significant associations with subsequent year high-cost patient indicators, even after adjusting for demographic, socioeconomic, medical conditions, and prior healthcare cost and utilisation.ConclusionsWe demonstrate that oral health measures are associated with an increased risk of becoming a high-cost patient.