When individuals with autism and intellectual and developmental disabilities exhibit severe problem behavior, assessment and treatment are often warranted. Parents of such individuals are at high risk for developing parenting stress. In this study, 194 parents completed the Parenting Stress Index-Short Form at their child's time of admission to and discharge from inpatient or outpatient treatment for severe problem behavior. Parent stress was examined in relation to rate and function of child problem behavior as determined via functional analysis. Repeated measures analyses of variance were conducted, and differential effects were observed when child participants exhibited an attention, automatic, or mands function for problem behavior. These findings highlight the importance of considering function of problem behavior in relation to parenting stress.