Understanding factors that influence the efficacy of functional communication training has both practical and conceptual benefits. The current study extended research in this area by exploring data from 95 consecutive applications of functional communication training with extinction across two independent clinics. We selected candidate predictor variables based on conceptual analysis, conducted preliminary exploratory analyses, and then selectively applied quantitative methods that are used in precision medicine to examine their accuracy and predictive utility. Treatment outcomes were better when challenging behavior was maintained by a single function than they were when it was maintained by multiple functions; however, these differences were most apparent among cases with an escape function. We also analyzed within-session responding to explore the potential influence of unprogrammed establishing operations on decrements in treatment efficacy. Our within-session measure only distinguished responders from nonresponders when escape was one of the multiple functions. Additional research is needed to validate these findings with an independent sample and to address a number of clinical conceptual issues.