The skills, social impairments and challenging behaviours of a total population of 166 children, with severe intellectual disabilities and/or autism, were assessed through interview with the main carers, when the children were under 15 years old (time 1). Twelve years later, 141 of these individuals were re-assessed, using the same measures (time 2). "Abnormal" behaviours tended to reduce with age and were associated with poorer language skills and poorer quality of social interaction. Individuals with most abnormal behaviours at time 1, tended to have most at time 2. Abnormal behaviour at time 2 was predicted by the presence of abnormal behaviour at time 1, poor expressive language at time 1, poor quality of social interaction at time 1 and a diagnosis of autism/autistic continuum at time 1.