Policies of fluid administration/restriction in critically ill patients have evolved over recent years. Abundant fluid resuscitation is encouraged during the early stage of severe sepsis. But a conservative fluid strategy is recommended in later stages, in particular when lungs are injured. Both strategies are risky if uncontrolled. Tests detecting volume unresponsiveness at any moment of fluid resuscitation or detecting volume unresponsiveness at any moment of fluid restriction would help to better assess the benefit/risk ratio of continuing such strategies. Measuring the short-term hemodynamic changes during passive leg raising can be reliably used for that purpose in both situations, even when patients are breathing spontaneously.