In a method and a mammography device, a virtual mask is created from an x-ray image of the breast for positioning the breast for a biopsy. The mask reproduces the image region covered by the breast tissue. A region targeted for the biopsy, and located within the breast tissue, is marked in the mask. The breast is then positioned for the biopsy in the mammography device between a support plate and a compression plate, the compression plate being provided with a recess for introducing the biopsy instrument therethrough. The mask and the compression plate are superimposed to cause the marked region to be located within this recess. The breast is then moved until the position and the contour thereof substantially coincide with the position and contour of the mask.