Continuous positive airway pressure (CPAP) and noninvasive ventilation (NIV) are frequently used inhospital for treating respiratory failure, especially in treatment of acute cardiogenic pulmonary edema and exacerbation of chronic obstructive pulmonary disease. Early initiation of treatment is important for success and introduction already in the prehospital setting may be beneficial. Our goal was to assess the evidence for an effect of prehospital CPAP or NIV as a supplement to standard medical treatment alone on the following outcome measures; mortality, hospital length of stay, intensive care unit length of stay, and intubation rate. We undertook a systematic review based on a search in the three databases: PubMed, EMBASE, and Cochrane. We included 12 studies in our review, but only four of these were of acceptable size and quality to conclude on our endpoints of interest. All four studies examine prehospital CPAP. Of these, only one small, randomized controlled trial shows a reduced mortality rate and a reduced intubation rate with supplemental CPAP. The other three studies have neutral findings, but in two of these a trend toward lower intubation rate is found. The effect of supplemental NIV has only been evaluated in smaller studies with insufficient power to conclude on our endpoints. None of these studies have shown an effect on neither mortality nor intubation rate, but two small, randomized controlled trials show a reduction in intensive care unit length of stay and a trend toward lower intubation rate. The risk of both type two errors and publication bias is evident, and the findings are not consistent enough to make solid conclusion on supplemental prehospital NIV. Large, randomized controlled trials regarding the effect of NIV and CPAP as supplement to standard medical treatment alone, in the prehospital setting, are needed.