The main aims of this study were to examine the adaptive behaviour of institutionalized mentally retarded persons and analyze the psychometric properties of the AAMD Adaptive Behavior Scale (ABS). The evolution of the concepts of mental retardation and adaptive behaviour were outlined in relation to historical development of the treatment and care of mentally retarded persons. The subjects of this study were the residents (n = 42I) of an institution for mentally retarded persons. Examination of the validity and reliability of ABS confirmed the mostly good quality of the scale. Factor analysis of ABS produced three factors, named Personal independence, Social maladaptation, and Personal maladaptation. Adaptive behaviour of the residents was analyzed against various background variables. The general level of Personal independence ,was very variable. Substantial deficits existed especially in profoundly retarded residents. Social maladaptation was most common among the moderately retarded, while Personal maladaptation was most frequent in the profoundly retarded. The study contains a separate, detailed analysis on self-injurious behavior.