Background: The behavioural function of handmouthing has been assessed across various studies utilising analogue functional analyses. The aim of the current study was to expand upon research on this relatively understudied behaviour by examining the relationship between handmouthing and "Gastro-Esophageal Reflux Disorder" (GERD), and the potential variables maintaining this challenging behaviour in 30 individuals with intellectual disability. Method: First, the relationship of GERD to handmouthing was examined. Second, the behavioural function of handmouthing was assessed with an indirect functional assessment, the "Questions About Behavioral Function" (QABF). Results: We found that a diagnosis of GERD was present with greater frequency among individuals with handmouthing than among matched peers who did not engage in handmouthing. Non-social reinforcement was most frequently endorsed as a behavioural function of this challenging behaviour. Discussion: Although individuals with handmouthing exhibited more GERD than those who did not handmouth, the findings suggest that handmouthing was most often maintained by non-social reinforcement. The importance of utilising functional assessment methodology for the assessment and treatment of handmouthing is discussed. (Contains 3 tables.)