['Tumour xenografts of well-discernible sizes can be examined well by molecular ultrasound. Here, we investigated whether very early breast carcinomas express sufficient levels of VEGFR2 for reliable molecular ultrasound imaging with targeted microbubbles.', 'MCF-7 breast cancer xenografts were orthotopically implanted in nude mice (n\u2009=\u200926). Tumours measuring from 4 mm(3) (2 mm diameter) up to 65 mm(3) (5 mm diameter) were examined with automated 3D molecular ultrasound using clinically translatable VEGFR2-targeted microbubbles (BR55). Additionally, the relative tumour blood volume was assessed with non-targeted microbubbles (BR38). In vivo ultrasound data were validated by quantitative immunohistochemistry.', 'Very small lesions 2 mm in diameter showed the highest binding of VEGFR2-specific microbubbles. In larger tumours significantly less BR55 accumulated (p\u2009=\u20090.023). Nonetheless, binding of VEGFR2-targeted microbubbles was still high enough for imaging. The relative blood volume was comparable at all tumour sizes. Both findings were confirmed by immunohistochemistry. Additionally, a significantly enhanced number of large and mature vessels were detected with increasing tumour size (p\u2009<\u20090.01), explaining the decrease in VEGFR2 expression during tumour growth.'] Does the high angiogenic activity in very early breast cancer enable reliable imaging with VEGFR2-targeted microbubbles ( BR55 )?