Vimentin was preferentially expressed in medullary and high grade ductal not otherwise specified (NOS) carcinomas. It was not expressed in lobular carcinoma whether of the classical or the variant types. In the present study, 58 cases of invasive breast carcinomas were tested for vimentin on formaldehyde fixed paraffin embedded sections. Vimentin was expressed in 14/ 44 (32%) of infiltrating duct carcinoma NOS. It was expressed in less than 10% of tumour cells in 5/44 (11.4%) and in > or = 10% of tumour cells in 4/7 (57%). However, none of the lobular carcinoma expressed vimentin. Vimentin was expressed in 9 of 18 (50%) of grade III infiltrating duct NOS carcinoma versus 5 of 21 (24%) of grade II and 0 of 5 (0%) of grade I carcinoma. It was preferentially expressed in tumour growing in broad anastomosing bands or sheets with numerous mitoses, high nuclear grade, scanty supportive stroma and extensive necrosis.