Little is known about long-term outcomes. We investigate the adaptive trajectories and their risk factors in ASD. Data were obtained from 281 children prospectively followed untill adulthood. The final sample consisted of 106 individuals. Vineland scores were collected at baseline (T1), 3 (T2), 10 (T3), and 15 (T4) years later. A group-based method was used to identify homogeneous patterns of adaptive skills trajectories. Results show that among the children initially categorized as autistic, 82.6% remained over the ADOS diagnostic threshold, 11.9% converted to atypical autism, and 5.4% fell under the ADOS threshold. Most atypical autism diagnoses were unstable. Most (81.7%) autistic participants had an ID at inclusion. At T1, 59.3% were nonverbal, but only 39% at T4. Most changes occurred between 4 and 8 years of age. Approximately 25% of participants exhibited a "high" growth trajectory, in which progress continues throughout adolescence, and 75% a "low" growth trajectory, characterized by greater autistic symptoms, intellectual disability, and lower language abilities reflected by high CARS scores, low apparent DQ, and speech difficulties, which mostly, but not always, predicted low trajectories. Our findings suggest that the adaptive prognosis of autism is mostly poor in this cohort, biased toward intellectual disability. However, changes in diagnostic, speech, and adaptive status are not uncommon, even for indivduals with low measured intelligence or apparent intellectual disability, and are sometimes difficult to predict. Autism Research 2018, 11: 1455-1467. © 2018 International Society for Autism Research, Wiley Periodicals, Inc. LAY SUMMARY: Most autism diagnoses given before 5 years of age are stable to adulthood, but one-fifth of individuals are no longer considered to be autistic, even in a cohort biased toward apparent intellectual disability. Conversely, atypical autism diagnoses are mostly unstable. One-third of children who are nonverbal at 5 years are verbal within 15 years, mostly before 8 years of age. Concerning adaptive behavior outcomes, only one-fourth of children exhibit a high-growth trajectory through at least 15 years.