The study of thrombin production included 68 patients with severe pneumonia (SP) undergoing monitoring plasma thrombin potential in the thrombin generation test. Thrombin production was found to decrease in the patients who died compared with those alive on days 3-5 and 7-10 after onset of the disease. Endogenous thrombin potential decreased progressively during the first 7-10 days among the patients with the fatal outcome of SP. This trend in thrombin generation can be used to predict the unfavourable outcome of SP.