Vimentin expression, growth fractions (GF), and estrogen receptor (ER) levels were determined for 90 untreated primary breast carcinomas. Coexpression of keratin and vimentin was found in approximately 20% of the tumors regardless of menopausal status. Vimentin was expressed preferentially in tumor cells of high-grade ductal breast carcinomas (15 of 28 histologic grade 3 vs. 0 of 40 grades 1 and 2). Vimentin expression was found preferentially in tumors with high GF (greater than 15% Ki-67 positive by immunoperoxidase staining) and low ER levels (less than 60 fmols/mg protein by a monoclonal enzyme immunoassay). Sixty-eight percent of tumors in this group were vimentin positive and 88% of all vimentin-positive tumors fell into this category. More than 50% of the tumor cells coexpressed vimentin and keratin. Thus, vimentin expression may be helpful in identifying a substantial subset of ER-independent breast carcinomas with poor prognostic indicators.