While interleukins (IL)-1alpha and beta are thought to play an important role in malignant disease, little is still known about their expression in breast cancer. We have used reverse transcription-polymerase chain reaction, enzyme-linked immunosorbent assay (ELISA), and immunohistochemistry (IHC) to analyze the expression of IL-1alpha and beta in breast cancer tissues, and compared their expression to estrogen receptor (ER) status and grading. In breast cancer cell lines, we found an inverse correlation between IL-1alpha and beta gene expression and differentiation, and only one highly invasive tumor cell line expressed IL-1alpha protein, while IL-beta was not detectable. Breast cancer tissue expressed variable amounts of IL-1alpha and beta messenger RNA, but consistently high levels of IL-1 type I receptor. IL-1alpha protein was detectable in malignant epithelium and adjacent stroma in 88% of cases. IL-1alpha expression was correlated with poor differentiation (P= 0.002; r= 0.469) and decreasing ERalpha expression (P= 0.004; r=-0.387). Stromal IL-1alpha was confined to areas with low or absent ERalpha protein expression in adjacent tumor epithelium (P= 0.001; r=-0.457). Taken together, we have demonstrated a functional IL-1 system in breast cancer and observed an inverse correlation between IL-1alpha and sex steroid receptor expression. We suggest that the expression of IL-1alpha in poorly differentiated, ERalpha-negative tumors contributes to their malignant phenotype.