The Short-Term Assessment of Risk and Treatability (START) assists risk assessment for seven risk outcomes based on scoring of risk and protective factors and assignment of clinically-informed risk levels. Its predictive validity for violence and self-harm has been established in males with schizophrenia, but accuracy across pathologically diverse samples is unknown. Routine START assessments and 3-month risk outcome data of N = 527 adult, inpatients in a UK secure mental health facility were collected. The sample was divided into diagnostic groups; predictive validity was established using receiver operating characteristics regression (rocreg) analysis in which potential covariates were controlled. In most single-diagnosis groups START risk factors ('vulnerabilities'), protective factors ('strengths'), and clinically-informed estimates predicted multiple risk outcomes with effect sizes similar to previous research. Self-harm was not predicted among patients with an organic diagnosis. The START risk estimates predicted physical aggression in all diagnostic groups, and verbal aggression, self-harm and self-neglect in most diagnostic groups. The START can assist assessment of aggressive, self-harm, and self-neglect across a range of diagnostic groups. Further research with larger sample sizes of those with multiple diagnoses is required.