Ten trauma patients and 13 burns patients were studied intensively for the first 36 h and subsequent 6 days post injury in order to investigate the mechanism of trauma and burn associated proteinuria. Burns patient's initial maximum proteinuria occurred between 4 and 8 h post injury, whilst trauma patients showed greatest proteinuria within 4 h. In both groups coexisting myoglobinuria or marked elevation of serum renin activity was not found during the first 36 h. Following admission serum lipid peroxides rose in burns patients reaching a maximum between 2 and 8 h following injury, after which levels fell rapidly; in contrast trauma patient's values were initially within normal limits, but rose sharply after 12 h with peaks occurring between 16 h and 7 days post injury. Some patients within both groups showed a recurrent protein leak 2 to 5 days post injury. The data suggest that myoglobinuria or raised renin levels do not play an important role in trauma associated proteinuria.