This study reports on cognitive abilities and attainment in 29 children and 21 adults with Smith-Magenis syndrome. There were 13 boys and 16 girls aged 6 to 16 years, and nine men and 12 women aged 16 to 52 years. All had mild to severe learning disabilities with no differences overall between verbal and performance skills, but with a particular profile of cognitive strengths and weaknesses. Levels of attainment and of adaptive behaviour were strikingly low, and the group of adults emerged as much more dependent on carers than might have been expected from their general level of intellectual functioning. Reasons for this discrepancy are explored in terms of the severe behavioural difficulties characteristic of the syndrome.