Background: Inpatient services for people with intellectual and other types of developmental disabilities (IDD) who also have forensic or risk issues are largely provided in secure hospitals. Although this is a health service sector with high levels of expenditure, there is limited empirical information on patient outcomes from such services. In order for a future substantive longitudinal outcomes study in forensic IDD services to be informed and feasible, more needs to be understood about the outcome domains that are of relevance and importance and how they should be measured. A preliminary series of studies was therefore undertaken. Objectives: To synthesise evidence in relation to the outcome domains that have been researched in the existing literature from hospital and community forensic services for people with IDD, within the broad domains of service effectiveness, patient safety and patient experience. To identify a definitive framework of outcome domains (and associated measures and indicators) based both on this research evidence and on the views of patients, carers and clinicians. To synthesise the information gathered in order to inform design of future multisite longitudinal research in the sector. Design: Three linked studies were conducted. Stage 1 was a systematic review and evidence synthesis of outcome domains and measures as found within the forensic IDD literature. Stage 2 was a consultation exercise with 15 patients with IDD and six carers. Stage 3 was a modified Delphi consensus exercise with 15 clinicians and experts using the information gathered at stages 1 and 2. Results: At stage 1, 60 studies that researched a range of outcomes in forensic IDD services were identified from the literature. This resulted in the construction of an initial framework of outcome domains. The consultation with patients and carers at stage 2 added to these framework domains that related particularly to carer experience and the level of support post discharge in the community. The Delphi process at stage 3 confirmed the validity of the resulting framework for clinician. This survey also identified the outcome measures preferred by clinicians and those that are currently utilised in services. Thus, indicators of appropriate measures in some important domains were identified, although there was a paucity of measures in other domains. Conclusions: Together, these three linked studies led to the development of an evidence-based framework of key outcome domains and subdomains. A provisional list of associated measures and indicators was developed, although with the paucity of measures in some domains development of specific indicators may be required. With further refinement this could eventually be utilised by services and commissioners for comparative purposes, and in future empirical research on outcomes in forensic IDD services. An outline research proposal closely linked to recent policy initiatives was proposed. Limitations of the study include the relatively small number of carers and patients and range of experts consulted. Future work: This would comprise a national longitudinal study tracking IDD in patients through hospitalisation and discharge. Study registration: This study is registered as PROSPERO CRD42015016941. Funding: The National Institute for Health Research Health Services and Delivery Research programme.