BACKGROUND: Older medical inpatients have complex biopsychosocial problems, which often lead to prolonged hospital stays. Proactive Integrated Consultation-Liaison Psychiatry was designed to help ward teams manage biopsychosocial complexity and thereby reduce the time that older medical inpatients spend in hospital. OBJECTIVES: To assess the experience, effectiveness and cost-effectiveness of enhancing medical care with Proactive Integrated Consultation-Liaison Psychiatry in The HOME Study. DESIGN AND SETTING: A parallel-group, multicentre, individually randomised controlled trial with process and economic evaluations in 24 medical wards of three National Health Service hospitals. PARTICIPANTS: Patients aged ≥ 65 years, admitted in an emergency and expected to remain in hospital for at least 2 days from the time of enrolment. INTERVENTIONS: Proactive Integrated Consultation-Liaison Psychiatry clinicians (consultation-liaison psychiatrists supported by assisting clinicians) made proactive biopsychosocial assessments of patients' problems, then delivered discharge-focused care as integrated members of ward teams. Usual care was provided by ward teams. PROCESS EVALUATION: Observations on training Proactive Integrated Consultation-Liaison Psychiatry clinicians and the care they provided; Proactive Integrated Consultation-Liaison Psychiatry clinicians' experiences of working in the new service model; patients' and ward staff members' experiences of Proactive Integrated Consultation-Liaison Psychiatry. PRIMARY OUTCOME: Time spent as an inpatient (during the index admission and any emergency re-admissions) in the 30 days post randomisation. SECONDARY OUTCOMES: Rate of discharge for the total length of the index admission; discharge destination; length of the index admission post randomisation truncated at 30 days; number of emergency re-admissions, number of days in hospital and rate of death in the year post randomisation; the patient's experience of the hospital stay and view on its length; anxiety; depression; cognitive function; independent functioning; health-related quality of life; overall quality of life. ECONOMIC EVALUATION: Cost-effectiveness of Proactive Integrated Consultation-Liaison Psychiatry over 1, 3 and 12 months from a hospital perspective. RESULTS: Two thousand seven hundred and forty-four participants were enrolled (1399 male, 1345 female; mean age 82.3 years; 2565 White; 1373 Proactive Integrated Consultation-Liaison Psychiatry, 1371 usual care). Proactive Integrated Consultation-Liaison Psychiatry was experienced positively by patients (43 interviews) and ward staff (54 interviews). The mean time spent in hospital in the 30 days post randomisation was 11.37 days (standard deviation 8.74) with Proactive Integrated Consultation-Liaison Psychiatry and was 11.85 days (standard deviation 9.00) with usual care; adjusted mean difference -0.45 (95% confidence interval -1.11 to 0.21; p = 0.18). The only statistically and clinically significant difference in secondary outcomes was the rate of discharge, which was 8.5% higher [rate ratio 1.09 (95% confidence interval 1.00 to 1.17); p = 0.042] with Proactive Integrated Consultation-Liaison Psychiatry - a difference most apparent in patients who stayed for > 2 weeks. Compared with usual care, Proactive Integrated Consultation-Liaison Psychiatry was estimated to be modestly cost saving and cost-effective over 1 and 3, but not 12, months. MAIN LIMITATION: Uncertain generalisability to other populations, hospitals and healthcare systems. CONCLUSIONS: This is the first randomised controlled trial of Proactive Integrated Consultation-Liaison Psychiatry. Proactive Integrated Consultation-Liaison Psychiatry is experienced by older medical inpatients and ward staff as enhancing medical care. It is also likely to be cost saving in the short term. Although the trial does not provide strong evidence that Proactive Integrated Consultation-Liaison Psychiatry reduces time in hospital, it does support and inform its future development and evaluation. FUTURE RESEARCH: Effectiveness of different forms of Proactive Integrated Consultation-Liaison Psychiatry; predictors of long hospital stays; barriers to and facilitators of the implementation of biopsychosocial medical care. STUDY REGISTRATION: This study is registered as Current Controlled Trials ISRCTN86120296. FUNDING: This award was funded by the National Institute for Health and Care Research (NIHR) Health and Social Care Delivery Research programme (NIHR award ref: 15/11/16) and is published in full in Health and Social Care Delivery Research; Vol. 13, No. 41. See the NIHR Funding and Awards website for further award information.