In patients affected with different tumours, disorders concerning clotting are frequently observed. The biological processes leading to coagulation are probably involved in the mechanisms of metastasis. We studied plasma levels of thrombin-antithrombin III complexes (TAT) in 90 patients affected with lung tumours subgrouped in small cell and non-small cell (NSC) lung cancer: 17 patients had no evidence of disease after surgery (NE); the remaining 73 patients were divided according to the absence (LOC) or the presence (META) of metastases. All the patients were followed up for several months. In all the lung cancer patient groups, at the beginning of the study we detected TAT levels that were higher than in controls. During the follow-up period, the NSC-NE patients with no recurrence of the disease as well as the NSC-LOC patients responding to the treatment had a decrease in TAT levels (p < 0.01 and p < 0.05, respectively). The NSC-META patients with progression of their disease had, in contrast, an increase in TAT levels (p < 0.01). Our data reveal the presence of 'latent coagulation disorders' as assessed by the presence of high TAT levels in the majority of lung cancer patients. The follow-up study indicates that in the NSC group, a relation exists between coagulation activation and rate of tumour progression and/or response to treatment. In cancer patients the early detection of coagulation disorders could also allow, therefore, the prevention of thromboembolism and/or haemorrhage by administration of appropriate treatment.