The value of plasma fibrinogen and antithrombin as predictors of severe sepsis was investigated in 69 adult hematological patients, who had altogether 93 periods of neutropenic fever. Patients had either acute myeloid leukemia or had received a high dose of chemotherapy supported by autologous stem cell transplantation. In febrile periods with severe sepsis, the median fibrinogen concentration at the start of the fever was significantly higher (5.0 g/L) than that without severe sepsis (4.5 g/L) (p=0.009). Normal plasma fibrinogen could rule out a group of patients with severe sepsis at the beginning of the fever. The antithrombin activity decreased, both in fever periods with severe sepsis and in those without. The decrease in antithrombin activity was found to be greater in fever periods characterized by severe sepsis. In conclusion, elevated plasma fibrinogen and constantly decreasing antithrombin were shown to be linked to the development of severe sepsis.