Background: A small proportion of patients account for most of the healthcare costs. Previous studies of supportive interventions have several methodological limitations and results are mixed. This article describes the protocol for Proactive Health Support: a national randomized controlled trial of telephone-based self-management support (ClinicalTrials.gov, NCT03628469). The main aim of the intervention is to reduce hospital admissions and improve quality of life at six months. Methods: A sample size of 4400 is needed and individuals with the highest risk of hospital admission in Denmark are invited by electronic communication and telephone to participate in a 1:1 randomized controlled trial. The intervention group receives one face-to-face start-up session followed by telephone sessions about individual goals regarding participants' knowledge, coping and need of healthcare. Quality of life was assessed with the mental health composite score of the SF-36v2 questionnaire. Primary analyses are done using the intention-to-treat principle. Discussion: The trial has been approved by The Regional Committee on Health Research Ethics (SJ-677). Intervention nurses do not assume clinical responsibility for the participants and the intervention is an addition to the general healthcare services. The intervention is complex due to challenging skills and behaviors required by nurses, individual tailoring of the intervention, and interacting intervention components. The study therefore includes process evaluation. The research program comprises: 1. Development initiation, 2. Intervention effect, 3. Cost-effectiveness, 4. Organizational implementation, and 5. Participants' experiences. Inclusion to the trial began April 9th, 2018, was completed July 1st, 2019 and follow-up will be completed February 1st, 2020.