Three-dimensional sonography of the mamma with a Voluson annular array transducer (10 MHz) (Kretztechnik, Austria) is a new method applicable in differential diagnosis of mamma foci. 50 patients (19 of them with breast cancer) were thus pre-surgically examined. Both sectional and stereoscopic representations were made use of. The suspected diagnoses and their correspondences with post-surgical findings were compared to the correspondences obtained through 2D-sonography. 3D-sonography produced 4 cases of the incorrectly positive diagnosis breast cancer, 2D-sonography 2 cases of incorrectly negative diagnoses. Other advantages of 3D-sonography over 2D-sonography include: better judgement of the conditional of focal environs (infiltration), existence and form of intracystic structures and of multifocal disease, short duration of examination, possible re-diagnosing of stored data.