Twenty-nine patients, 15 to 85-year-old (mean: 50 years) who presented with a pleural effusion after trauma were studied. The blood content of pleural fluid was confirmed by thoracocentesis. None of the patients had been taking anticoagulant drugs during the fortnight preceding the trauma. Thoracocentesis was always carried out less than 90 min after the trauma. The following parameters were measured in the haemothorax liquid samples: clotting fibrinogen fraction (Fg C), fibrin degradation D-dimers, functional plasminogen, alpha 2-antiplasmin, alpha 2-macroglobulin, plasminogen tissue activator (tPA Ag), type 1 tPA plasma inhibitor (PAI), and haematocrit. Haemothorax liquid haematocrit values ranged from 13 to 35% (25 +/- 7%, with a mean peripheral venous haematocrit of 34 +/- 6%). Only three patients had some Fg C (0.05-0.13 g.l-1). The D-dimer level was very high (0.23 +/- 0.22 g.l-1). The other factors involved in fibrinolysis were also present. Moreover, there was a statistically significant inverse correlation between D-dimer and alpha 2-macroglobulin levels (r = -0.64, p less than 0.0025). These data suggest two possible mechanisms to explain the fibrinogen levels: coagulation is activated, followed by an important fibrinolytic reaction elicited by the large amounts of plasminogen and tPA present in the haemothorax liquid.