Interest in the differential diagnostic capabilities of instruments designed to measure adaptive behavior has greatly increased in recent years. The purpose of the present investigation was to examine the differential diagnostic potential of the Scales of Independent Behavior (SIB). It was hypothesized that a multiple discriminant function analysis, with SIB cluster domain scores as independent variables, would lead to the correct group classification of 479 subjects (303 nonretarded, 54 educable mentally retarded (EMR) and 122 trainable mentally retarded (TMR) subjects). A linear combination of scores from the four SIB cluster domains explained 63% of the variance between groups and correctly classified 76% of the subjects by group membership. Findings support the use of the SIB as a significant contributor to the process of differential diagnosis, with concomitant implications for improving decision-making practices pertinent to classification for service and program placement purposes.