Introduction. Many patients who undergo cardiac surgery assisted by cardiopulmonary bypass (CPB) suffer cerebral injury, and microemboli are thought to play a role. Because increased duration of CPB is associated with increased risk of subsequent cerebral dysfunction, we investigated whether cerebral microemboli were also more numerous with longer duration of CPB.Methods. Brain specimens were obtained from 36 patients who died within 3 weeks after CPB. They were embedded in celloidin, sectioned 100 μm thick, and stained for alkaline phosphatase, which outlines arterioles and capillaries. In such preparations, emboli can be seen as swellings in the vessels. Cerebral microemboli were counted in equal areas and scored as small, medium, or large to estimate the embolic load (volume of emboli).Results. With increasing survival time after CPB, the embolic load declined (p < 0.0001), as the emboli pumped slowly through the brain. Also with increasing time after CPB, the percentage of large and medium emboli declined (p = 0.0034). The emboli broke into smaller globules as they passed through the capillaries. Longer duration of CPB was associated with increased embolic load (p = 0.0026). For each 1-hour increase in the duration of CPB, the embolic load increased by 90.5%.Conclusions. Thousands of microemboli were found in the brains of patients within a few days after CPB, and within a week, most had cleared from the brain. Increasing duration of CPB was associated with increasing embolic load. The scavenging of pericardial blood during surgery is a potential source of lipid emboli, and prolonged CPB may be associated with increased scavenging of blood. Thus, the removal of lipid emboli from scavenged blood may be important to reduce post-CPB cerebral dysfunction. It is not known why increased time on CPB results in more lipid cerebral emboli. Introduction. Many patients who undergo cardiac surgery assisted by cardiopulmonary bypass (CPB) suffer cerebral injury, and microemboli are thought to play a role. Because increased duration of CPB is associated with increased risk of subsequent cerebral dysfunction, we investigated whether cerebral microemboli were also more numerous with longer duration of CPB. Methods. Brain specimens were obtained from 36 patients who died within 3 weeks after CPB. They were embedded in celloidin, sectioned 100 μm thick, and stained for alkaline phosphatase, which outlines arterioles and capillaries. In such preparations, emboli can be seen as swellings in the vessels. Cerebral microemboli were counted in equal areas and scored as small, medium, or large to estimate the embolic load (volume of emboli). Results. With increasing survival time after CPB, the embolic load declined (p < 0.0001), as the emboli pumped slowly through the brain. Also with increasing time after CPB, the percentage of large and medium emboli declined (p = 0.0034). The emboli broke into smaller globules as they passed through the capillaries. Longer duration of CPB was associated with increased embolic load (p = 0.0026). For each 1-hour increase in the duration of CPB, the embolic load increased by 90.5%. Conclusions. Thousands of microemboli were found in the brains of patients within a few days after CPB, and within a week, most had cleared from the brain. Increasing duration of CPB was associated with increasing embolic load. The scavenging of pericardial blood during surgery is a potential source of lipid emboli, and prolonged CPB may be associated with increased scavenging of blood. Thus, the removal of lipid emboli from scavenged blood may be important to reduce post-CPB cerebral dysfunction. It is not known why increased time on CPB results in more lipid cerebral emboli.