Background: The study concerns the unknown value of group instruction for mothers of young children with Down syndrome (DS) in preventing or minimizing sleep problems. Method: (1) Children with DS were randomly allocated to an Instruction group (given basic information about children's sleep) and a Control group for later comparison including objective sleep measures (actometry); and (2) As a secondary exercise, parental reports of sleep problems and objective sleep measures were compared in children with DS and a small sample of unimpaired children (U group) of comparable age and sex distribution. Results: Instruction (shown to have improved mothers' knowledge) was associated with improvement in behavioural sleep problem ratings at 6-month follow up with no change in sleep-related breathing problem ratings or actometry findings. Sleep-related breathing problem symptoms and actometry abnormalities were significantly more common in the DS group than in the U group at baseline, with no obvious differences for behavioural sleep problems. Conclusions: Group instruction offers some benefit regarding behavioural sleep problems but not for sleep-related breathing problems to which more attention should be paid in children with DS, with a view to precise diagnosis and treatment.