Background and objectives: Ultrasound is a common and necessary part of acute care medicine, but may present an infection risk to patients secondary to transfer of infectious agents between patients. Our primary objective was to detect blood contamination on ultrasound equipment in emergency departments (EDs) and intensive care units. Secondary objectives included detection of microbial contamination and determination of factors associated with contamination. Design and setting: We tested ultrasound equipment used in five EDs and five ICUs for blood and microbial contamination, and collated and analysed contamination data using tables and multiple logistic regression. Main outcome measures and results: We performed 109 tests for blood and 131 tests for microbial contamination, with 61% of samples testing positive for blood contamination (95% CI, 52%–71%) and 48% testing positive for microbiological contamination (95% CI, 40%–57%). Transducer leads and transducers had high blood contamination (88% and 57%, respectively) and microbiological contamination (62% and 46%, respectively). Equipment from ICUs was less likely to test positive (odds ratio, 0.55; 95% CI, 0.37–0.79). Only 51% of blood-contaminated samples were visibly stained, and visible staining was not associated with microbiological contamination (57%; P=1). Conclusion: Our results show significant contamination of ultrasound equipment, and that visual inspection of equipment is neither sufficient nor reliable in excluding contamination. Ultrasound equipment is a possible factor in the transmission of infectious diseases in EDs and ICUs. Guidelines must be formulated, disseminated and rapidly adopted to ensure the safety of the most acutely ill patients exposed to ultrasound procedures in acute care settings.