Because abnormalities in hemostatic factors may in part account for the risk of stroke and thromboembolism in atrial fibrillation, we measured plasma fibrinogen and fibrin D-dimer levels in 33 patients (18 men and 15 women, mean age 60.8 +/- 1.4 years [mean +/- SEM]) with paroxysmal atrial fibrillation (PAF) and 12 patients (3 men and 9 women, mean age 51.0 +/- 4.2 years) with paroxysmal supraventricular tachycardia (PSVT). Levels of these markers were compared to levels in (1) patients with chronic atrial fibrillation; (2) hospital controls (age-matched [age +/- 5 years] and sex-matched patients in sinus rhythm with coronary artery disease and normal left ventricular function); and (3) healthy population controls in sinus rhythm. Patients with PAF had intermediate levels of median plasma fibrinogen and fibrin D-dimer when compared to patients with chronic atrial fibrillation and controls in sinus rhythm (both p < 0.001). There was no relation with atrial size or ventricular function on echocardiography. Patients with PSVT had plasma fibrinogen and fibrin D-dimer levels that were similar to the median levels of the population controls, suggesting that there was no excess in thrombogenesis. These findings are consistent with the hypothesis that atrial fibrillation is related to the increases in plasma fibrinogen and fibrin D-dimer levels. Patients with PAF have intermediate levels of these markers, a finding that is consistent with the intermediate risk of thromboembolism in such patients.