High rates of behavior and emotional problems have been consistently reported in children and adolescents with autism. Elevated rates of mental health problems have also been reported in adults with autism. Little is known, however, about the longitudinal development of behavior and emotional problems in autism. This study followed a cohort of children and adolescents over 18 years. Outcomes were evaluated in terms of behavior and emotional problems and autism symptomatology. The role of childhood factors (age, gender, IQ, behavior, and emotional problems) and the environment (socioeconomic disadvantage) were considered in terms of adult outcomes. Overall, improvements in comorbid behavior and emotional problems and autism symptomatology were observed. However, rates of comorbid behavior and emotional problems in adulthood remained high.