Although gagging has a profound effect on the delivery of dental care, it is a relatively under-investigated phenomenon. This study aimed to derive a prevalence estimate of gagging during dental treatment based on patient-reported information, to determine some socio-demographic and psychological correlates and to assess the relationship of gagging with self-reported oral health and avoidance of dental care. Data were collected with a survey among Dutch twin families (n = 11 771). Estimated overall prevalence of gagging during dental treatment was 8·2% (95% CI 7·7-8·7). Patients' self-report of gagging was found to be significantly associated with female sex, a lower level of education and higher levels of dental trait anxiety, gagging-related fears (e.g. fear of objects in the mouth), anxious depression and neuroticism. Gagging also appeared to be significantly associated with untreated cavities, gingival bleeding and wearing full dentures, but not with avoidance of dental care. It can be concluded that individuals who report to gag during dental treatment are moderately dentally anxious, fear-specific situations that can trigger a gagging response and, albeit visiting the dentist equally frequently, report to have a poorer oral health compared to those who do not gag.