Purpose <p>Increasing numbers of individuals diagnosed with autism spectrum disorder (ASD) in childhood are now reaching adulthood, yet knowledge of their long-term outcomes remains limited. This study evaluated adult clinical characteristics, outcomes, and prognostic factors in individuals diagnosed with ASD in childhood.</p> Methods <p>Eighty-seven adults participated, including five (5.7%) in whom loss of autism diagnosis (LAD) was identified. The remaining participants (<i>n</i> = 82) were classified by their initial childhood diagnoses: autistic disorder (49.4%), pervasive developmental disorder–not otherwise specified (20.7%), and Asperger’s syndrome (24.7%). Adult outcomes were assessed using a structured functional and psychosocial rating system.</p> Results <p>Subtypes differed significantly in age at first sentence, regression history, age at diagnosis, and initiation of special education. Individuals with Asperger’s syndrome and LAD had higher university attendance and graduation rates. Overall outcomes were very poor (5.7%), poor (55.2%), fair (24.2%), good (8%), and very good (6.9%), with significant group differences. Poor outcomes were significantly more frequent among individuals with intellectual disability (ID), regression history, co-occurring psychiatric conditions, and older age at first sentence. ID and regression significantly predicted poor outcomes, while absence of ID predicted LAD.</p> Conclusion <p>Findings highlight the heterogeneity of ASD trajectories across the lifespan. While some individuals lose the diagnosis, outcomes vary widely depending on individual characteristics and diagnostic subtype. ID and regression emerged as key predictors for poorer outcomes. These findings offer clinically relevant insights by clarifying prognostic markers and long-term outcomes, informing diagnostic assessment and intervention planning, and emphasizing the need for tailored support strategies.</p>