Summary.We have previously reported high levels of the coagulation inhibitor TFPI in the blood of patients with gastrointestinal cancer. TFPI is not an acute-phase reactant, but high levels have also been reported in patients with septicaemia and disseminated intravascular coagulation (DIC). To study its relationship with other types of malignancy, TFPI activity was first determined in plasma samples from 214 patients with various malignancies. In a second cohort of 83 patients, total and free TFPI antigen, protein C, antithrombin, fibrin monomer and D-dimer were also measured. Elevated TFPI activity and antigens were found in about half of the patients with solid tumours. In contrast, elevated TFPI was rare in haematological malignancies (12%). In the 18 patients with acute nonlymphocytic leukaemia (ANLL), elevated free TFPI was found only in patients who also had DIC. No correlation was found between TFPI levels and fibrin monomer or D-dimer levels. Only four out of 20 patients with solid tumours had normal levels of fibrin monomer and D-dimer, yet three out of these four had elevated TFPI. In conclusion, elevated TFPI in ANLL is related to the coexistence of DIC. In solid tumour disease increased TFPI may reduce protective fibrin formation, but the pathogenic mechanism is as yet unknown.