While there is laboratory evidence that the activity of recombinant activated factor VII (rFVIIa) is reduced by the presence of acidosis and hypothermia, there is limited clinical data to support this observation. Recombinant FVIIa may be used as rescue therapy in surgical patients who have bleeding that is refractory to conventional therapy. However, these patients are also frequently acidotic and hypothermic at the time the drug is administered. In this retrospective study, the records of 38 adult surgical patients who received rFVIIa intraoperatively or within six hours postoperatively were reviewed. The requirements for red cell transfusion in the two hours following the administration of rFVIIa and the need for repeated doses of rFVIIa were recorded. The relationship between red cell transfusion and pH and temperature of the patient at the time of rFVIIa administration was assessed by multiple regression analysis. The major finding was an inverse relationship between the degree of acidosis at the time of rFVIIa administration and the requirement for either subsequent blood transfusion or repeat dosing of rFVIIa (P=0.003 and P <0.001 respectively). For patients with a pH <7.2 vs. pH =7.2, the odds ratio for receiving two or more packs of red blood cells within two hours of rFVIIa administration was 15:1. This effect was not observed for hypothermia. The implication of this study is that rFVIIa may be less effective when administered to severely acidotic patients. Further studies are required to examine whether this is related to the acidosis directly, or is secondary to other intraoperative variables affecting acidosis. The clinical utility of rFVIIa in acidotic patients also requires further investigation.