We measured fibrinogen levels as well as the fibrinolytic parameters tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor 1 (PAI-1) in plasma samples obtained at basal conditions and after stimulating the fibrinolytic system by venous occlusion (VO). Samples were taken from patients with primary pulmonary hypertension (PPH), with secondary thromboembolic pulmonary hypertension (SPHTH), with secondary pulmonary hypertension due to congenital heart disease with Eisenmenger's reaction (SPHCD), and from healthy control individuals (CON). Fibrinogen levels were not significantly different between the groups with PPH and SPHTH or between SPHCD and CON. The latter groups, however, exhibited significantly lower fibrinogen plasma levels compared with PPH or SPHTH (p < 0.01). Basal plasma levels of t-PA antigen, t-PA activity, and PAI-1 activity, respectively, did not differ significantly between the study groups. After VO, mean t-PA activity levels increased to a higher extent in control subjects compared with patients with PPH, or SPHTH, or SPHCD, with significant differences only between CON and SPHTH or CON and PPH (p < 0.03). Patients with PPH and SPHTH exhibit both increased fibrinogen plasma levels and a diminished fibrinolytic response compared with healthy subjects. Moreover, the fibrinogen plasma levels in patients with SPHCD are in normal range, and the fibrinolytic response is similar to CON compared with PPH and SPHTH, thus indicating the existence of a comparable prothrombotic situation in patients with PPH and SPHTH.