The atypical antipsychotic medication aripiprazole was evaluated using a randomized AB multiple baseline, double-blind, placebo-controlled design for the treatment of severe problem behavior with 4 children with intellectual and developmental disabilities. Functional analysis (FA) was conducted concurrent with the medication evaluation to determine whether severe problem behavior was differentially affected by the medication during assessment situations associated with specific reinforcement contingencies. Weekly rating scales were completed by parents or teachers. Descriptive and visual analysis indicated that aripiprazole had some apparent differential effects across behavioral function and behavioral topography for 3 out of 4 participants. This study provides a further demonstration of how FA can generate information relevant to understanding medication effects on the reinforcement contingencies maintaining severe problem behavior. (Contains 1 table and 5 figures.)