The construct validity of four dimensions of adaptive and maladaptive behavior was investigated using the multitrait-multimethod matrix procedure of Campbell and Fiske (1959). Measures of four traits--cognitive competence, social competence, social maladaption, and personal maladaption--were obtained on a sample of 157 persons with moderate, severe, or profound mental retardation using each of three methods of measurement--standardized assessment instrument, day shift staff ratings, and evening shift staff ratings. Applying the Campbell and Fiske rules of thumb and recently proposed structural equation modeling techniques to the data demonstrated strong convergent validity, clear discriminant validity, and only moderate levels of method variance in the observed measures. Implications of the results for the assessment of adaptive behavior and its dimensional structure were discussed.