This prospective study examined the developmental changes over time of adolescents diagnosed in toddlerhood with autism spectrum disorder and searched for child characteristics at toddlerhood that predict outcome at adolescence. The study included 65 participants who were divided into low cognitive (developmental quotient [less than] 85; N = 41) and high cognitive (developmental quotient [greater than or equal to] 85; N = 21) groups in adolescence. Participants underwent a comprehensive assessment of cognitive ability, adaptive skills, and autism severity. Significant differences in the current clinical phenotypes and in developmental changes over time were found between the two cognitive groups. At baseline, the high cognitive group had significantly less severe social communication deficits. Only the high cognitive group showed a decrease in social communication deficits over time. Although the two groups did not differ in their adaptive skills at the time of diagnosis, the high cognitive group had better adaptive skills at adolescence. Better adaptive communication skills in toddlerhood were associated with better outcome at adolescence in cognitive ability, adaptive skills, and fewer autism symptoms. Less impaired baseline social affect and better cognitive ability predicted higher cognitive level and adaptive skills at adolescence. Cognitive potential of individuals with autism spectrum disorder plays an important role in long-term outcome and comprehensive evaluations at toddlerhood have strong prognostic value in adolescence.