Previous research has demonstrated that individuals with a dual diagnosis often engage in challenging behavior as a means to fulfil their needs and wants. Functional behavioral assessments (FBA) are a way of evaluating these behaviors and creating effective interventions to reduce them and increase socially appropriate alternative behaviors. The present study assessed the convergent validity for three types of FBAs, including the Questions About Behavioral Function (QABF) questionnaire, trial-based functional analysis, and traditional functional analysis, for three adults with a dual diagnosis within a day program setting. Results found correspondence between two forms of assessments, the trial-based functional analysis and traditional functional analysis, for one out of the three subjects. Due to a lack of engagement in the targeted behavior for the other two subjects, results were inconclusive. Results of the QABF did not match those of the functional analyses for any of the subjects, suggesting that the assessment should be used with caution. The trial-based functional analysis may be a viable tool for assessing function for the challenging behavior of adults with a dual diagnosis, although it should not be used as a replacement for the traditional functional analysis.