A review is presented of the investigations carried out concerning the adaptive behavior of persons with the dual disability of mental retardation and autism/PDD. A close correspondence is found between the results obtained by means of a Dutch set of scales, the SRZ, SGZ, and SMZ, and those obtained by means of the Vineland Adaptive Behavior Scales. Compared with matched nonautistic persons, the performance of dually disabled persons is found to be particularly poor in the domain Social Skills/Socialization and somewhat less poor in the domain Communication. Performance of the two categories does not differ in the domains Self Help/Daily Living Skills and Gross Motor Skills/Motor Skills. As a net result, persons of the dually disabled category are found to obtain comparatively low total scores for social competency, the SRZ-Total/Adaptive Behavior Composite. In addition, autistic mentally retarded persons are found to display more maladaptive behavior. For comparative studies in the mental retardation field the use of level of Self Help/Daily Living Skills and, to a lesser extent, level of Gross Motor Skills/Motor Skills are recommended as suitable matching variables.