Introduction Prehospital emergency medicine is a challenging discipline characterized by a high level of acuity, a lack of clinical information and a wide range of clinical conditions. These factors contribute to the fact that prehospital emergency medicine is a high-risk discipline in terms of medical errors. Prehospital use of Computerized Decision Support System (CDSS) may be a way to increase patient safety but very few studies evaluate the effect in prehospital care. The aim of the present study is to evaluate a CDSS. Methods In this non-blind block randomized, controlled trial, 60 ambulance nurses participated, randomized into 2 groups. To compensate for an expected learning effect the groups was further divided in two groups, one started with case A and the other group started with case B. The intervention group had access to and treated the two simulated patient cases with the aid of a CDSS. The control group treated the same cases with the aid of a regional guideline in paper format. The performance that was measured was compliance with regional prehospital guidelines and On Scene Time (OST). Results There was no significant difference in the two group's characteristics. The intervention group had a higher compliance in the both cases, 80% vs. 60% (p < 0.001) but the control group was complete the cases in the half of the time compare to the intervention group (p < 0.001). Conclusion The results indicate that this CDSS increases the ambulance nurses' compliance with regional prehospital guidelines but at the expense of an increase in OST.