Acinetobacter baumannii causes severe infections (pneumonia, bacteremia, organ space) with high lethality in hospitalised critically ill patients. It can acquire resistance to all classes of antibiotics (multidrug resistance, MDR) except an 'old' drug, colistin, which may be the only therapeutic option. However, colistin is not registered for this indication. The addition of rifampicin to colistin has been shown to be synergistic in vitro, and may be promising in vivo, but this combination has not been studied in comparison with colistin alone.