Data were collected, using interviews with parents and teachers and rating scales, concerning the behaviour of two groups of mentally retarded children: 193 children with Down's Syndrome (DS) and 154 children with a similar degree of handicap were identified in the same schools. The proportions of children who had high scores on the Rutter or an additional behaviour check list, were similar in the two groups. Deviant behaviour was markedly more common in both sets of retarded children than in their siblings next in age; 31% of the children with DS and 29% of controls were judged to be well adjusted, while 38% of the DS children and 49% of the controls had significant behaviour disorder. Conduct disorders were most common in the children with DS. ‘Psychosis' was the most common diagnostic label in the control group, but was also found in the children with DS.