Background There has been no consensus about the prevalence and mechanism of generation of microbubbles in mechanical prosthetic heart valves (PHV). The aim of this study was to determine the prevalence of microbubbles in PHV and their relation to serum lactic dehydrogenase (LDH) levels. Methods and Results We prospectively studied 150 normally functioning mitral PHV (98 bileaflet and 52 monoleaflet) in 150 patients with the use of transesophageal echocardiography with a multiplane 5-MHz probe. None of the patients had an aortic prosthetic valve. Blood was drawn to determine serum LDH level. None of the patients had any factors that might affect the LDH level other than the PHV-related hemolysis. Patients with spontaneous echo contrast in the left atrium that might affect the assessment of the microbubbles were excluded. We devised a method to determine the amount of microbubbles for each PHV. Microbubbles were detected in 118 (79%) of 150 PHV, including 97 (99%) of 98 bileaflet valves and 21 (40%) of 52 monoleaflet (tilting disk) valves (P < .0001). Intraobserver variability was not statistically significant for the determination of the amount of microbubbles (z = 1.7, P = .08). There was a strong correlation between serum LDH levels and the amount of microbubbles (r_s = 0.69, P < .001). Conclusions Microbubbles were detected in more patients than reported previously. They were found to be associated more with the bileaflet than the monoleaflet PHV. Sorin monoleaflet valves were associated with microbubbles significantly less often than the others. There was a strong correlation between serum LDH levels and microbubble counts, which suggests that hemolysis may be related to microbubble formation. (Am Heart J 1999;137:463-8.)