Young persons with intellectual disabilities and challenging behaviour in transition usually have complex needs, which may not be served well within existing resources. In this article, we present a survey of all the young people, between 16 and 18 years of age with intellectual disabilities and challenging behaviour identified in one inner London borough. They were in transition to adult services at the time of the study (between 2006 and 2008). The objective was to examine their socio-demographic and clinical characteristics, pattern of service use and associated costs of care. An assessment toolkit was devised to measure the mental and physical health, challenging behaviour and service use of the sample. Instruments within the toolkit included the Strengths and Difficulties Questionnaire, challenging behaviour scale, Client Service Receipt Inventory (CSRI) and socio-demographic data form. Twenty-seven individuals in transition to adult services had challenging behaviour, 23 of whom had mental health diagnoses and 18 of whom had physical diagnoses. Severity of challenging behaviour did not correlate with cost of care. Informal care accounted for the highest proportion of the total cost of care (66%) with education being the second largest contributor at 22%. Evidence on transition outcomes for young people with complex needs and intellectual disabilities and associated costs is lacking. This article illustrates some of the relevant issues in this area. Further research is required to investigate these aspects and guide commissioning of appropriate services.