Abstract Microbubbles are very useful as ultrasound (US) contrast agents because of their excellent scattering properties. Because microbubbles of different sizes can be used for this purpose, the contrast images produced by different-sized microbubbles are studied in this paper. The contrast images from microbubbles of average sizes 35.5 μm and 2.1 μm were investigated experimentally. Although a low concentration of microbubbles produces contrast-enhanced images without artefacts, an excess of microbubbles results in distorted images. From experimental observation, the distortion of an image caused by microbubbles of average size 35.5 μm was mainly due to multiple scattering, and that by 2.1-μm microbubbles was due to the acoustic shadowing effect. With the use of the tissue-mimicking phantoms of known acoustical properties, the brightness of the contrast images from the microbubble suspension was calculated. The calculated and experimental results of the contrast images produced from microbubbles of average size 35.5 μm were closer to each other when there was no image distortion. When image distortion caused by multiple-scattering occurred, the experimental pixel brightness was higher. For smaller microbubbles of average size 2.1 μm, calculated results of free microbubbles showed a weaker contrast effect than the experimental results. By taking the effect of the coatings of microbubbles into consideration, the calculated brightness of contrast images became much closer to the experimental one.