This is a study of plasma HBP -levels of a previously published trial of G-CSF in critically ill patients (Pettila et al. Critical Care Medicine 2000). The original study was a prospective, randomised, double-blind, placebo-controlled trial of filgrastim in patients with acute respiratory failure requiring intubation. In this substudy, the investigators evaluated the effect of filgrastim on HBP -concentrations in critically ill patients.