OBJECTIVE Anticoagulation therapy is strongly recommended in all patients with mitral stenosis (MS) in atrial fibrillation (AF) but this treatment is controversial in patients in sinus rhythm (SR). The objective of the present study was to investigate the coagulation activity in patients with MS in sinus rhythm compared to those in atrial fibrillation. METHODS The authors studied the levels of biochemical markers of thrombin generation (thrombin-anti-thrombin [TAT] complex,fibrinogen, and factor XIII) and fibrinolysis (D-dimer) in specimens of blood from the atria in 35 consecutive patients with moderate to severe MS (18 in sinus rhythm and 17 in AF) who underwent percutaneous balloon mitral valvotomy. RESULTS The levels coagulation factors in left atrium in patients with MS in SR and AF were thrombin-anti-thrombin complex = 77.21 +/- 8.87 mg/L vs. 73.48 +/- 7.78 mg/L, p = 0.755, fibrinogen = 356.57 +/- 41.86 mg/L vs. 271.62 +/- 22.47 mg/L, p = 0.089, factor XIII = 139.88 +/- 8.96 mg/L vs. 123.42 +/- 6.24 mg/L, p = 0.152, and D-dimer = 846.14 +/- 137.84 mg/L vs. 693.88 +/- 164.67 mg/L, p = 0.481. Levels of coagulation activities did not correlate with the left atrial size. CONCLUSIONS This present study demonstrates that coagulation activity is not different whether they are in SR or in AF and suggests that anticoagulation therapy should be considered in these patients.