The aim of this study was to investigate which factors that influences decision making among Swedish ambulance nurses in emergency care situations. Nurses in ambulance are sometimes forced to make decisions without adequate information. Data collected from interviews with 14 ambulance nurses was analyzed. The informants described 30 incidents during which they had to make fast decisions. A qualitative content analysis of the text was made, from which different categories were identified. It was found that when nurses are called to an emergency, the extent and degree of difficulty of the incident is decisive for how decisions are made. In addition, the nurses’ experience is important for decision making, because the experience factor constitutes a qualitative difference between a novice nurse and a more experienced nurse’s influence on decision-making. Furthermore, external factors, such as the uncertainty of a prehospital environment, expectations and pressures from an environment in which one is working while being observed by other people, and collaborating with many different operators, all contribute to making decisions in an urgent situation even more complex. Further studies are needed to understand the complexity of decision making in emergency situations.