The influence of mammography upon stage classification according to the TNM system is evaluated in 590 breast cancer patients. By using tumour size measured on the radiograph instead of the tumour diameter determined by palpation it was possible to reclassify a significant proportion of cases in stage III. As much as 23% (52/228) of cases of stage III could correctly be grouped into stage II or I. The prognostic significance of this change is evident from the 5-year survival rates. The cases, removed from stage III by correction of the tumour size, show the same 5-year survival rates (65%) as cases primarily classified into stage I (64%) and II (63%). In view of these results we do recommend the use of mammography for stage classification, because it allows correction with prognostic significance.