Background <p>Research consistently demonstrates significantly higher rates of mental health disorders among incarcerated individuals compared to the general population. This paper examines the prevalence of mental health disorders among all individuals serving preventive detention in Norway between 2009 and 2022, and compares the prevalence of mental health disorders in this group with a comparison group of individuals convicted of serious violent and sexual offences, and with the total prison population.</p> Methods <p>We conducted a registry-based study to investigate the prevalence of psychiatric morbidity within this population in Norwegian prisons, using a subset of the data from the nPRIS-cohort including all people imprisoned in Norway between January 1st, 2000, and December 31st, 2022 (<i>n</i> = 112,877). The cohort, including 202 individuals serving preventive detention in the period 2009–2022, were linked to the Norwegian Patient Registry (NPR) and socio-economic data from Statistics Norway.</p> Results <p>The prevalence of mental health disorders was high across all three comparison groups. However, of the preventive detention population (<i>n</i> = 202) almost all (95.0%) had at least one mental health diagnosis during the study period. In addition, 73.8% of the preventive detention population had received a severe mental health disorder diagnosis during the observation period (compared to 23.7% for the comparison group and 28.3% for the total prison population). Approximately one third of the preventive detention population had a combination of three or more of the five most common diagnoses.</p> Conclusion <p>The preventive detention population in Norway is clearly a group with significant mental health treatment needs – even when compared to a prison population that is itself showing increasing prevalence of mental health disorders, indicating that they represent a particularly vulnerable population within the prison system.</p>