This study aimed to examine the effectiveness of The Enhanced Support Service (ESS) pilot in reducing custodial violence and disruption, and the associated costs, by observing the behavioural change of the 35 service users who participated in ESS intervention within its first 22 months of operation. Frequencies of recorded incidents of aggressive behaviours, self-harming behaviours, noncompliance, and positive behaviours were counted from routine administrative systems using a coding structure developed in previous studies. The count data were analysed using nonparametric tests and Poisson regression models to derive an Incident Rate Ratio (IRR). Findings suggest the ESS is associated with a reduction in aggressive behaviours and noncompliance, with medium to large effect sizes ( r = .31-.53); however, it was not associated with a reduction in deliberate self-harm or increased positive behaviours. The Poisson models revealed that levels of pre-intervention behaviour, intervention length, intervention completion, and service location had varying effects on postintervention behaviour, with those who completed intervention demonstrating more favourable outcomes. The ESS service model was associated with a reduction in behaviour that challenges, which has implications for the reduction in associated social, economic, and political costs-as well as the commissioning of interventions and future research in this area.