A stereotaxic instrument is described which facilitates percutaneous needle biopsy of the breast for microscopic diagnosis. The instrument consists of an x-ray tube mounted on a hinged arm, pivoting stand, patient table, compression device with film holder and scale system, coordinate-controlled puncture device with separate stand, biopsy instrument, and calculator. The sampling site can be located at a precision within +/- 1 mm. The instrument can also be used for positioning of metal and dye indicators for guiding surgery and for postoperative identification of excised tumors.