This study on sensitivity and attachment included 55 toddlers and their parents. Samples included children with autism spectrum disorder (ASD), mental retardation, language delay, and typical development. Children were diagnosed at 4 years of age. Two years before diagnosis, attachment was assessed with the Strange Situation procedure, and parental sensitivity and child involvement during free play were assessed with the Emotional Availability Scale. Parents of children with ASD were equally sensitive as parents of children without ASD, but their children showed more attachment disorganization and less child involvement. More sensitive parents had more secure children, but only in the group without ASD. Less severe autistic symptoms in the social domain predicted more attachment security. Autism challenges the validity of attachment theory.