Both experimental and clinical data have shown that coagulation disorders are common in patients with cancer although clinical symptoms occur rarely. A prethrombotic state is probably involved in the mechanism of metastatic spread. Anticoagulant treatment, with either warfarin or heparin, has been shown to have a positive influence in small cell lung cancer. The purpose of this study was to evaluate the prethrombotic state as a possible marker of the outcome of lung cancer. Pretreatment prothrombin time (PT), partial thromboplastin time (PTT), antithrombin III (AT-III), platelet blood count (P), fibrinogen (F) and D-dimer (DD) were prospectively recorded in a series of 286 consecutive patients with a new primary lung cancer. Other recorded variables (32 in all) consisted of a set of anthropometric, clinical, physical, laboratory, radiological and pathological data. All patients were carefully followed up, and their subsequent clinical course recorded. Spearman rank correlation tests between coagulation factors were weakly significant, or more often non-significant. The best correlation index was that between PT and PTT (ra = -0.25). Univariate analyses of survival showed that a prolonged value of PT (P = 0.00167) and higher values of F (P = 0.00143) and DD (P = 0.0005) were associated with a poor prognosis. A few, weak relationships between well-known prognostic variables and coagulation abnormalities were also found. Because of the weakness of this correlation pattern, coagulation factors emerged in all the Cox's regression analyses as important predictors of survival, regardless of the number and type of cofactors used. A prethrombotic state (depicted by a prolongation of PT and increase of DD) is confirmed in this study as an aggravating condition in lung cancer. Studies attempting to reverse possible haemostatic abnormalities with the use of anticoagulants are justified by the present data.