Background: Understanding the operant function of self-injurious behavior in autistic individuals is imperative to understanding its etiology and designing effective prevention strategies. Methods: We conducted a postal survey of residential care facilities including two functional assessment tools: QABF and IfES. Questionnaires for N = 178 residents were returned. Results: For 21.2% a single, and for 62.9% multiple functions were identified by the QABF, with the most frequent function being "escape" (51.8%). The IfES identified "unable to cope with a situation" as primary antecedent for 39.8% of the residents. Non-vocal individuals presented a higher number of functions and had overall higher subscale scores. Conclusion: The study was the first to investigate single and multiple functions of SIB within a large sample of autistic adults. Results suggest that multiple functions of SIB are common with negative reinforcement contingencies being most important. Results have to be interpreted with caution due to psychometric limitations of the instruments used.