Introduction: Current research findings in the field of intellectual disabilities (ID) regarding the relationship between mental health problems and challenging behavior are inconclusive and/or contradictory. The aim of this study was to further investigate the putative association between these two highly prevalent phenomena in people with ID, and specifically to explore the hypothesis that challenging behaviors may be behavioral equivalents of mental health problems. Methods: A sample of 160 adults accessing secondary care ID health services was assessed using five validated measures. These included ratings of severity of disability, mental health problems, autism behaviors, physical health problems, and four different aspects of challenging behavior. In conjunction with demographic information, four multiple regression analyses were undertaken to examine the interaction between mental health problems (moderated by severity of disability) and ratings of overall challenging behavior, aggression, self-injurious behavior, and stereotypy. In each case, age, gender, autism, and physical health problems were included as covariates. Results: There was a statistically significant association between mental health problems and ratings of overall challenging behavior, as well as the moderating effect of severity of disability. Importantly, the positive association between mental health problems and challenging behavior was only significant at more severe levels of disability. Conclusions: These findings support the "behavioral equivalents" hypothesis for mental health problems and challenging behaviors. However, further longitudinal research is required before this hypothesis can be considered unequivocally supported.