We report on the development and behaviour of three young children with Smith-Magenis syndrome (SMS), del 17p11.2. The behaviour problems and the psychomotor delay in preschool children with SMS are often more striking than the dysmorphic features and can serve as a useful clue to the diagnosis. We compare the behaviour with reported data. The behaviour problems in the three four year olds include very demanding behaviour, severe temper tantrums, hyperactivity, aggressive behaviour, self injurious behaviour, sleeping problems and stereotypic behaviour. Head banging, hand, wrist or finger biting are present. Onychotyllomania is not observed. Insertion of objects in the mouth as well as excessive nose picking is very frequent, although polyembolokoilomania is not present. The so called self hug when excited is present in one child. The behaviour problems and psychomotor delay represent a major management problem for the parents.