Telephone nurses give advice and support and make assessments based on verbal communication only. Web-based decision support systems are often used to increase patient safety and make medically correct assessments. The aim of the present this study was to describe factors affecting the use of a decision support system and experiences with this system among telephone nurses in Swedish primary health care. Observations and semistructured interviews were conducted. Six registered nurses with at least 1?year of experience of telephone nursing participated. Field notes and interviews were analyzed by qualitative content analysis. The main findings of the present this study were factors that decrease the decision support system use or promote deviation from decision support system use, factors that are positive for decision support system use and the decision support system complicates the work. Underuse and deviations from decision support systems can be a safety risk, because decisions are based on too little information. Further research with observations of telephone nurses' use of decision support systems is needed to develop both telephone nursing and decision support systems.