This paper presents the routine implementation of a method for use of three-dimensional (3-D) treatment planning for breast cancer patients with node negative disease, treated with breast conserving surgery. The patients were positioned on a device which fitted a CT aperture with a diameter of 70 cm. A patient reference system was defined by the mamillary plane and the sternum midline. Eight to twelve CT images per patient were taken. The target volume was defined in several CT slices. The treatment planning was made with a 3-D treatment planning system; the treatment technique was isocentric with tangential fields. The patient position was the same at the CT scanner and during the treatment. Four to six portal films per field per patient were routinely taken. The reproducibility of the set-up was investigated on 20 consecutive patients: the treatment-to-treatment variation results were related to the margin of the target. The comparison between treatment planning and portal films gave information about the isocenter displacement. No large discrepancy between measurements on simulator film and on portal films was found. Based on the presented results the described treatment technique has been introduced as part of a routine clinical practice at our institution.