At the scene of an accident, the most severely injured patients need trauma care urgently. Bystanders are often present before the emergency medical service arrives and may be able to limit trauma-related damage by providing trauma care at the scene. The aim of this prospective study conducted in Mainz, Germany, and Vienna, Austria, was to compare the frequency and quality of bystander trauma care in moderately versus severely injured patients. Five specific measures (making the scene readily visible for oncoming traffic, extrication and positioning of the trauma patient, control of haemorrhage, and hypothermia protection) were assessed in a questionnaire and evaluated statistically. Bystanders were present at the scene in 58.7% of all accidents. Making the scene readily visible for oncoming traffic, patient extrication and patient positioning were initiated significantly more often than haemorrhage control and hypothermia protection. Extrication, patient positioning and hypothermia protection were initiated significantly more often in moderately (NACA I-II) compared to severely (NACA III-VII) injured patients. In severely injured patients, bystanders attempted measures less frequently and the measures performed were more often incorrect compared to those in moderately injured patients. Our findings show that severely injured patients received less and less appropriate bystander trauma care than moderately injured patients. In an effort to correct this serious problem and to improve trauma care on-scene, we advocate offering lay persons more extensive training in bystander trauma care.