The Functional Analysis Screening Tool (FAST) is a 16-item questionnaire about antecedent and consequent events that might be correlated with the occurrence of problem behavior. Items are organized into 4 functional categories based on contingencies that maintain problem behavior. We assessed interrater reliability of the FAST with 196 problem behaviors through independent administration to pairs of raters (Study 1). Mean item-by-item agreement between pairs of raters was 71.5%. Agreement for individual items ranged from 53.3% to 84.5%. Agreement on FAST outcomes, based on comparison of informants' highest totals, was 64.8%. We assessed the validity of the FAST by comparing its outcomes with results of 69 functional analyses (Study 2). The FAST score predicted the condition of the functional analysis in which the highest rate of problem behavior occurred in 44 cases (63.8%). Potential uses of the FAST in the context of a clinical interview, as well as limitations, are discussed.