Recent research findings suggest that idiosyncratic variables can influence the outcomes of functional analyses (E. G. Carr, Yarbrough, & Langdon, 1997). In the present study, we examined idiosyncratic environment—behavior relations more precisely after identifying stimuli (i.e., a particular toy and social interaction) associated with increased levels of problem behavior. Two children, an 8-year-old boy with moderate mental retardation and a 5-year-old boy with no developmental delays, participated. Results of functional analyses for both children indicated that idiosyncratic antecedent stimuli set the occasion for occurrences of problem behavior (hand biting or hand flapping) and that problem behavior persisted in the absence of social contingencies. Further analyses were conducted to identify specific components of the stimuli that occasioned problem behavior. Treatments based on results of the analyses successfully reduced self-injury and hand flapping.