Trained assistance for the anaesthetist appears likely to improve safety in anaesthesia. However, there are few objective data to support this assumption, and the requirement for a trained assistant is not universally enforced. We applied a simulation-based model developed in previous work to test the hypothesis that the presence of a trained assistant reduces error in anaesthesia. Ten randomly selected anaesthetists, five trained anaesthetic technicians and five theatre nurses without training in anaesthesia participated in two simulated emergencies, with anaesthetists working alternately with a technician or a nurse. The mean (SD) error rate per scenario was 4.75 (2.9). There were significantly fewer errors in the technician group than the nurse group (33 vs 62, p = 0.01) and this difference remained significant when errors were weighted for severity. This provides objective evidence supporting the requirement for trained assistance to the anaesthetist, and furthermore, demonstrates that a simulation-based model can provide rigorous evidence on safety interventions in anaesthesia.