Introduction: Despite adequate pre-transport stabilization, the physiological status of critically ill patients may deteriorate during transport. The aim of the study was to evaluate the incidence of potentially life threatening adverse events during helicopter transport.Methods: We retrospectively collected prehospital and transport data of consecutive NACA ≥ 4 patients, treated and transported by the emergency helicopter team (EMT) of Christophorus 2 over the period from 1.1.1994 to 30.9.1995. The EMT always used a strategy of stabilizing vital functions before transport. Adverse effects were classified as respiratory (arrest, cyanosis, aspiration), circulatory (arterial pressure, arrhythmia, cardiac arrest, bleeding), vomiting and other complications.Results: Over the 21 month period a total of 995 NACA ≥ 4 patients were transported. Adverse events occured in 39 transportations (3.92%).Cardio-circulatory: n=22, vomiting: n=15, respiratory: n=2, other complications: n=3. More than one adverse event occured in three cases, in one patient futile CPR was withdrawn during transport. Occurance rate of complications during helicopter flight increased in patients with higher NACA-scores, in medical patients compared to trauma victims (4.9% vs. 2.2%) and in interhospital transfers compared to trauma actions (4.9% vs. 3.6%).All adverse effects could be treated sufficiently during the short transportation period (mean 10.57 minutes) without flying interruption. Three patients experienced vomiting despite antiemetic therapy.Discussion: Adverse events during a helicopter flight are not unexpected. Adequate pre-transport care -on scene or in hospital- and a skilful transportation by an experienced team help to avoid complications. Vomiting can be regarded a problem during helicopter flight, which could turn out as a life threatening complication in a non-intubated patient. In patients at risk early intubation as well as the prophylactic use of antiemetic drugs should be considered.