Evidence of risk assessment procedures is scarce and inconclusive. The aim of this study is to evaluate the effects of risk assessment on aggression and the use of coercive interventions in an acute psychiatric admission setting. In addition, we evaluated nurse behaviour before and after the use of risk assessment. To take the fluctuations with regard to aggression and coercive interventions into account, we allowed 26 weeks for baseline measurements, followed by a 26 weeks steady-state period after the implementation of the risk assessment instrument. Contrary to expectations, no positive effects of risk assessment were found on aggression or on coercive interventions. Time spent in seclusion increased significantly with more than 10 hours on average after implementation. Furthermore, there were only negative effects on nurse behaviour and experiences. Among other things, they felt more stressed, spent more time on administration tasks and spent less time with patients after the implementation. In conclusion, there is insufficient evidence to use structured short-term risk assessment to reduce aggression or coercive interventions.