There are few well-controlled, published evaluations of sleep interventions for children with developmental disabilities. This paper evaluates a parent training programme using behavioural principles to reduce sleep problems in children with autism or fragile X syndrome (FXS). Training included bedtime routines, reinforcement, effective instructions, partner support, and extinction (removing reinforcement to reduce a behaviour). Programme efficacy was demonstrated by using a multiple baseline across-participant design. Social validity was also assessed. Five children with autism and one with Asperger syndrome (four males, two females; mean age 5y 6mo; age range 3y 5mo to 7y 4mo) and their parents; and seven children with FXS (six males, one female; mean age 4y 9mo; age range 1y 11mo to 9y 1mo) and their parents participated. Ten families completed intervention within the multiple baseline design. Most parents' goals were achieved and visual analysis of the sleep data indicated improvement. Settling problems, night waking, and co-sleeping were effectively reduced. The programme had high social validity and outcomes were clinically significant and maintained at follow-up.