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High intensity focused ultrasound (HIFU) treatment of soft tissues has been shown to result in a hyperechoic region in B-mode ultrasound (US) images. This is believed to result from bubble activity at the HIFU focus. Here, we report our in vivo results of detecting inertial and stable cavitation in correlation with the appearance of a hyperechoic region. The ultrasound system consisted of an HIFU transducer (3.3 MHz), a broadband A-mode transducer for active and passive cavitation detection (ACID and PCD), and US-imaging probe that were all co-focal and synchronized. HIFU, at in situ intensities of 220-1,710 W/cm2, was applied for 10 s to pig muscles in vivo at it focal depth of 2 cm. ACD and PCD results showed a strong correlation between the onset of cavitation and the appearance of a hyperechoic region. PCD results showed that inertial cavitation typically occurred prior (within 0.5 s) to the appearance of a hyperchoic region. The observed cavitation activity suggests that bubbles are present during the formation of a hyperechoic region at the HIFU focus.