Transition from a normal to a cancerous state is marked by alterations in the cytoskeletal structure of those cells involved. We have examined such changes to determine if these transitions are markers of disease progression. Cytokeratin (CK) protein and messenger RNA (mRNA) expression were examined in malignant and benign breast tissues. Flow cytometric results demonstrated a significant correlation between cytokeratin protein expression detected by 5D3 antibody, specific for cytokeratins 8, 18, and 19 and axillary node metastasis (P β«Ψβ¬ 0.01). A threshold of positivity of 338,000 molecules/cell was determined and reflected the wide range in cytokeratin levels expressed by normal or benign tissues. Examination of cytokeratins 8, 18, and 19 revealed a consistent pattern of expression with respect to tumor grade. Only cytokeratin 19 showed significant correlation with increasing tumor size (P β«Ψβ¬ 0.006). mRNA expression for cytokeratin 8 was significantly higher in node-positive compared with node-negative disease (P β«Ψβ¬ 0.02). Cytokeratin 18 mRNA levels were significantly lower in both node-negative (P β«Ψβ¬ 0.03) and node-positive (P β«Ψβ¬ 0.02) patients when compared with benign samples. Increased levels of cytokeratin 18 mRNA showed an inverse relationship with protein expression (P β«Ψβ¬ 0.05). The results indicate that cytokeratin expression in breast cancer may be associated with tumor progression. Furthermore, the alteration in the expression of individual cytokeratins deserves further investigation to determine the consequences of these changes with respect to cellular function.