The use of factor XI concentrates has been associated with thrombosis. Plasma markers of coagulation activation were measured before and 30, 60, 120 and 240 min after six infusions of the BPL factor XI concentrate. Five studies were completed before surgical intervention, one was undertaken in a patient with an intracerebral haemorrhage. Significant elevation of levels of fibrinopeptide A (FpA) (P < 0.05) and thrombin-antithrombin (TAT) were demonstrated following six infusions and prothrombin fragment F1.2 following four. Levels of all three markers had risen 60 min following concentrate administration and FpA levels remained elevated throughout the study period. Levels of D-dimer rose in four patients at 240 min. These results indicate significant thrombin generation by 60 min and subsequent plasmin generation consistent with coagulation activation by the factor XI concentrate. The greatest elevation of activation markers was seen in those subjects with pre-existing coagulation activation. We advise caution in the use of these products and awareness of the risks in patients who may already have activated coagulation states.