Objectives: Use of a rubber dam during root canal treatment is considered the standard of care because it enhances patient safety and optimises the odds of successful treatment. Nonetheless, not all dentists use a rubber dam, creating disconnect between presumed standard of care and what is actually done in clinical practice. Little is known about dentists' attitudes towards use of the rubber dam in their practices. The objectives were to: (1) quantify these attitudes and (2) test the hypothesis that specific attitudes are significantly associated with rubber dam use. Setting: National Dental Practice-Based Research Network (NationalDentalPBRN.org). Participants: 1490 network dentists. Outcome measures: Dentists completed a questionnaire about their attitudes towards rubber dam use during root canal treatment. Three attitude scales comprised 33 items that used a 5-point ordinal scale to measure beliefs about effectiveness, inconvenience, ease of placement, comparison to other isolation techniques and patient factors. Factor analysis, cluster analysis and multivariable logistic regression analysed the relationship between attitudes and rubber dam use. Results: All items had responses at each point on the 5-point scale, with an overall pattern of substantial variation across dentists. Five attitudinal factors (rubber dam effectiveness; inconvenient/time-consuming; ease of placement; effectiveness compared to Isolite; patient factors) and 4 clusters of practitioners were identified. Each factor and cluster was independently and strongly associated with rubber dam use. Conclusions: General dentists have substantial variation in attitudes about rubber dam use. Beliefs that rubber dam use is not effective, inconvenient, timeconsuming, not easy to place or affected by patient factors, were independently and significantly associated with lower rubber dam use. These attitudes explain why there is substantial discordance between presumed standard of care and actual practice.