Children with hyperactivity and self-destructive behaviour present a difficult problem for parents and paediatricians. The syndrome described by Smith and Magenis is due to a deletion on the short arm of chromosome 17: del(17)(p11.2 p11.2). Clinical manifestations include brachycephaly and a flat mid-face; brachydactyly; short, broad hands; mental retardation; and aberrant behaviour, including hyperactivity. We report on five children, and review the literature on a newly recognised syndrome in which the behaviour manifestations may precede and often overshadow the learning disabilities and unusual appearance. In addition, we have found sleep disturbance to be a major feature in our patients.