Factor analyses were performed utilizing the subscale scores of the AAMD [American Association on Mental Deficiency] Adaptive Behavior Scale, Part One, for 3354 institutionalized mentally retarded children and adults. Eight different age groups were studied to determine the underlying dimensions of adaptive behavior measured by the scale, similarity of factor structure across ages, whether there were developmental changes revealed by factor scores and the extent to which the above findings would be related to level of retardation. Three salient factorial dimensions (Personal Self-Sufficiency, Community Self-Sufficiency and Personal-Social Responsibility) appeared across a wide span of age ranges from childhood through senility. The implication of these factors was discussed in terms of the critical period of development, rate of growth and maximum level of growth of subjects grouped by level of retardation.