The purpose of this study was to determine the prevalence of microbubbles (MBs) in patients with prosthetic mitral valves (PMVs). The clinical and echocardiographic predictors of MB were investigated. We also analyzed the temporal relation between MBs and the thrombolytic treatment of thrombotic PMV dysfunction. The study material comprised 307 transesophageal echocardiography examinations in 279 patients (170 women and 109 men with a mean age of 37.9 +/- 13.3 years) with PMV. The PMV was mechanical in 245 patients (tilting-disk valves in 129, and bileaflet aortic valves in 116) and bioprosthetic mitral valves in 34 patients. Twenty-eight sessions of thrombolytic treatment were performed because of the obstructive (n = 18) and nonobstructive (n = 10) thrombi involving the PMV. No MBs were seen in any of the bioprosthetic valves. The MBs were present in 128 of 227 (56.4%) PMV without obstruction compared with only 1 of 18 (5.5%) valves with thrombotic obstruction (P <.0001). The MB were documented in 75.4% of the normal bileaflet valves compared with 38.5% of the tilting-disk valves (P <.0001). The MB intensity score was also significantly higher in the bileaflet valves (2.0 +/- 0.8 vs 0.7 +/- 0.7, P <.05). The incidence of MBs increased from 5.5% to 68.7% after successful thrombolysis in patients with obstructive PMV thrombi (P <.001). There were no other predictors of MBs in this series. The passage of MBs in the aortic root was not documented in any instances. We conclude that MBs are normal echocardiographic findings depending on the type and function of the mechanical PMVs.