The Vineland Adaptive Behavior Scales were used to assess adaptive behavior of MA-matched preschool children with autism or Down syndrome. Preschoolers with Down syndrome attained higher age equivalents and standard scores in the Socialization domain than did children with autism. Adaptive standard scores were unrelated to chronological age (CA) in both groups. However, adaptive age equivalent was positively related to CA for children with Down syndrome in all domains, but only in Communication for children with autism. All children with higher mental ages (MAs) also had higher levels of adaptive skills in all domains. Vineland standard scores recalculated by MA were higher in the Down syndrome than the autism group for Socialization. Results suggest that differing patterns of adaptive development are present very early in children with autism or Down syndrome.