In the previous issue of Critical Care, Vermeulen Windsant and colleagues demonstrate that transfusion of packed red cells is associated with a transient increase in plasma free haemoglobin and scavenging of nitric oxide in vitro. They also demonstrate that older units of blood have more free haemoglobin in their supernatants. Whether the administration of older stored blood results in adverse clinical outcomes is a topical clinical question. The present study proposes a mechanism for transfusion-related harm but also has implications for patients who have other sources of free haemoglobin in their circulation.