Single gas-containing pyogenic liver abscesses in 11 patients were studied by ultrasound and computed tomography (CT). On ultrasound, all abscesses were predominantly echogenic compared to the normal liver parenchyma. The gas collections appeared as hyperreflective areas arranged in clusters associated with acoustic shadowing and ring-down artifacts. Ten abscesses (90%) had ill-defined margins on ultrasound, causing underestimation of their sizes in these patients. All abscesses were shown to be multiloculated and had clearly defined borders on CT, not appreciated or mistaken for multiplicity of abscesses on ultrasound. Ultrasound may be inadequate in the evaluation of gas-containing liver abscesses, as they have complex echotexture in addition to ring-down artifacts, acoustic shadows and poorly-defined margins; leading to underestimation of abscess size, difficulty in identifying loculations and erroneous interpretation of multiplicity of abscess cavities.