The cytokine interleukin-1 (IL-1) can inhibit growth of breast cancer cells in culture and promote cellular differentiation in synergism with other growth factors. A secreted IL-1 receptor antagonist (sIL-1ra) has been described and an intracellular version (icIL-1ra) has been cloned; both antagonists block IL-1-dependent responses. We compared mRNA expression of IL-1 and both receptor antagonists in normal and neoplastic endometrium. RNA was extracted from five benign endometrial and five endometrial cancer whole-tissue specimens, reverse transcribed into cDNA, then amplified by polymerase chain reaction using specific primers for IL-1 alpha, IL-1 beta, sIL-1ra, and icIL-1ra. IL-1 alpha and IL-1 beta were expressed in variable amounts in all tissues; there was no difference in expression between normal and cancer specimens. In contrast, high levels of icIL-1ra were expressed in four of five cancer specimens compared with none of five normal tissues (P = 0.02). There was no expression of sIL-1ra in cancer and normal tissues. These preliminary experiments suggest that IL-1 is ubiquitously expressed in endometrial tissues whereas endometrial cancer preferentially expresses icIL-1ra. IcIL-1ra may regulate IL-1-mediated events such as growth and differentiation in endometrial neoplasia.