The single-item predictive validity of the Short-Term Assessment of Risk and Treatability (START) has not been thoroughly investigated, although this has great clinical relevance for the selection of treatment targets. Furthermore, it remains unclear whether the characteristic START additions of scoring strengths next to vulnerabilities and selecting key items, add incremental predictive validity. Finally, predictive validity has primarily been studied in inpatient settings and included mainly patients with a psychotic disorder. We analysed data from a mixed diagnostic sample of 195 forensic psychiatric outpatients with a 3-month and 170 patients with a 6-month follow-up period, using logistic regression analysis. The occurrence of violent or criminal behaviour was established based on the case manager’s recordings in the patient’s file. Only 5 of the 20 START items were found to have predictive validity: Impulse Control, Attitudes, Material Resources, Rule Adherence and Conduct. The last three were the only items for which incremental predictive validity was found with respect to scoring it as a strength and a vulnerability. Selection of key items did not add to the predictive validity. While possibly having therapeutic significance, the scoring of strength next to vulnerability and the selection of key items, may not be beneficial for risk assessment.