Two hundred sixty-two invasive breast carcinomas dating from 1979 to 1984 were tested for vimentin and keratin on formaldehyde-fixed paraffin-embedded sections. None of 26 lobular carcinomas expressed vimentin. Vimentin expression in 10% or more of tumor cells was found in 78% of medullary (14 of 18), in 16% of ductal not otherwise specified (NOS) (35 of 214), and in two of four mucinous carcinomas. A further seven tumors showed vimentin expression in less than 1% to 10% of the cells. Vimentin was expressed in tumor cells of 30% (28 of 93) of grade III invasive ductal NOS carcinomas versus 7% (7 of 105) of grade II and 0% of grade I carcinomas (0 of 10). Vimentin was found to be preferentially expressed in tumors growing in broad, often anastomosing bands or sheets with extensive necrosis, scanty supportive stroma, high nuclear grade, and numerous mitoses. The authors conclude that vimentin is not detected in lobular carcinomas, but is preferentially expressed in medullary and in high-grade ductal NOS breast carcinomas.