Background  This study investigates the role of IQ, autistic traits and challenging behaviours in affecting adult outcomes among adolescents who receive special educational assistance. Methods  A total of 58 participants were recruited from an ongoing longitudinal study. All received assessments of IQ, behavioural patterns (using the Childhood Behaviour Checklist – CBCL) and autistic traits (using the Social Communication Questionnaire – SCQ) during adolescence and were followed up 6 years later (at a mean age of 22 years) using the World Health Organization Disability Assessment Schedule II (WHO‐DAS II) to assess functional outcome. Results  A significant positive relationship was found between CBCL score and WHO‐DAS II score (β = 0.511, P = 0.001). IQ score showed a negative relationship with total WHO‐DAS II score (β = −0.247, P = 0.04). SCQ score was not found to significantly influence total WHO‐DAS II score (β = −0.028, P = 0.84). Conclusions  Although the role of global intellectual ability is important, these results stress the highly predictive value of adolescent behaviours on functional outcomes in adult life among young adults receiving special educational assistance.