Reliability and validity data are reported for an instrument designed to identify variables maintaining self-injurious behavior. The Motivation Assessment Scale (MAS) is a 16-item questionnaire that addresses the situational determinants of self-injurious behavior in persons with autism and other developmental disorders. The reliability study indicated that teachers of 50 developmentally disabled persons could agree on the variables presumably maintaining their student's self-injury (interrater reliability), and that they would be in agreement again 30 days later (test-retest reliability). The validity study indicated that teacher's ratings on the MAS of 8 subjects' self-injury predicted how their students would behave in analogue situations. Specifically, the MAS predicted the subjects' self-injurious behavior in situations with decreased adult attention, with increased academic demands, with restricted access to tangibles, and in unstructured settings. The MAS is presented as an alternative or adjunct to more formal functional analyses in efforts to identify the variables controlling self-injurious behavior.