The efficiency and reliability of an ultrasonographic technique for evaluating mammary neoplasms was tested in 19 female dogs with palpable tumours. A 7.5 MHz linear-array ultrasound transducer was used, with an aqueous stand-off pad between the probe and the skin. The ultrasonographic images were used to evaluate the shape, size and echogenicity of the mammary lesions, and their relationship with other tissues. The tumours were excised and analysed histologically. A comparison of the ultrasonographic and histological findings revealed that the ultrasonographic images of nine of the 11 malignant tumours had irregular margins and were polymorphous in shape, all 11 were heterogeneous in their internal echogenicity, seven had acoustic shadowing and three showed acoustic enhancement. In contrast, seven of the eight benign tumours had regular margins and were spherical or oval in shape, all eight were homogeneous in their internal echographic pattern, seven had edge shadowing, and all eight showed acoustic enhancement. Moreover, six of the 11 malignant neoplasia were invasive, whereas all the benign tumours were isolated.