The ratio of concentrations of pro-adrenomedullin (pro-ADM)/pro-endothelin (pro-END) immunoreactivity in body fluids of critically ill patients is used as for the diagnosis, course control and prognosis, including an assessment of the mortality risk, of severe life threatening diseases. Further, a treatment of critically ill patients having high levels of pro-ADM but insufficient levels of pro-END immunoreactivities with a medicament comprising vasoconstrictive endothelin or its precursors, and/or endothelin agonists or adrenomedullin antagonists is provided.