The study was performed in St. Petersburg in 2015 and comprised 34 children with diagnosed Down's syndrome (DS) aged 9-15 (mean 11) years (17 boys, 17 girls) who attended special schools. Control group was made up of 34 clinically healthy normal intelligence schoolchildren matched for age, sex and geographical distribution. The mothers were requested to fill in the Child Sleep Habit Questionnaire (CSHQ). The teachers were approached with the Achenbach Child Behaviour Check List and the Diagnostic Adaptive Behaviour Scale (DABS). Children with DS had significantly higher scores on reported sleep problems. The most prominent behavioural disturbance was on DSM-oriented attention-deficit/hyperactivity problems scale. DS children presented with lower scores on all DABS subscales and total DABS score. The values of IQ and the total sleep disturbance score were significant predictors of attention-deficit/hyperactivity problems, and the values of IQ and of the attention-deficit/hyperactivity were significant predictors of adaptive skills.