Background: The etiology of flank pain sometimes experienced during the administration of ultrasound contrast agents is unknown. The aim of this study was to investigate whether microbubble ultrasound contrast agents are retained within the renal microcirculation, which could lead to either flow disturbance or local release of vasoactive and pain mediators downstream from complement activation. Methods: Retention of lipid-shelled microbubbles in the renal microcirculation of mice was assessed by confocal fluorescent microscopy and contrast-enhanced ultrasound imaging with dose-escalating intravenous injection. Studies were performed with size-segregated microbubbles to investigate physical entrapment, after glycocalyx degradation and in wild-type and C3-deficient mice to investigate complement-mediated retention. Urinary bradykinin was measured before and after microbubble administrations. Renal contrast-enhanced ultrasound in human subjects (n = 13) was performed 7 to 10 min after the completion of lipid microbubble administration. Results: In both mice and humans, microbubble retention was detected in the renal cortex by persistent contrast-enhanced ultrasound signal enhancement. Microbubble retention in mice was linearly related to dose and occurred almost exclusively in cortical glomerular microvessels. Microbubble retention did not affect microsphere-derived renal blood flow. Microbubble retention was not influenced by glycocalyx degradation or by microbubble size, thereby excluding lodging, but was reduced by 90% (P < .01) in C3-deficient mice. Urinary bradykinin increased by 65% 5 min after microbubble injection. Conclusions: Lipid-shelled microbubbles are retained in the renal cortex because of complement-mediated interactions with glomerular microvascular endothelium. Microbubble retention does not adversely affect renal perfusion but does generate complement-related intermediates that are known to mediate nociception and could be responsible for flank pain.