Demographic variables and behavioral characteristics of institutionalized mentally retarded clients exhibiting both self-injurious behavior (SIB) and stereotypy, stereotypy alone, SIB alone, or neither of these behaviors were analyzed. Overall, there were no significant differences for the demographic variables measured. Multivariate analyses revealed that severity and frequency of behavior, sensory handicap, and sex of the subject were the best predictors of group membership. Moreover, this research suggests that SIB and stereotypy can be classified as stereotyped SIB and withdrawal stereotypy, respectively. Thus, the treatment modalities presently being applied to these behaviors could be inappropriate. Additional evidence is discussed that supports the belief that an organic physiological substrate or mechanism could be related to these aberrant behavior patterns, which would necessitate a new diagnostic classification and alternative forms of treatment.