Platelet activation occurs in peripheral blood of patients with rheumatic mitral stenosis (MS) and atrial fibrillation (AF) and could be related to abnormal thrombogenesis. However, the plasma level of platelet microparticles (PMPs), a marker of platelet activation, in patients with valvular AF remains unresolved. We performed a case control study of 20 patients with chronic rheumatic MS and chronic long AF and 10 healthy volunteers who were in sinus rhythm. Peripheral plasma levels of PMP were assessed by quantifying CD41-positive microparticles by flow cytometry. The mitral valve area was calculated by means of the Doppler pressure half-time method. The levels of circulating PMP were elevated significantly in valvular AF patients when compared with controls (12.36+/-5.51 x 10(5)/ml versus 7.56+/-0.86 x 10(5)/ml, P=0.011). Correlation analysis demonstrated that there was a significantly direct relationship between the severity of MS and PMP levels (r=-0.507, P=0.004). We conclude that in patients with rheumatic MS and AF, there is evidence of platelet activation detected by high PMP levels which have a significantly direct relationship with the severity of MS.