Leukemia‐inhibitory factor (LIF) is an inflammatory cytokine with pleiotropic activities. LIF was originally described as a differentiation factor of a murine leukemia cell line and was subsequently found to possess a broad spectrum of biological functions. Although LIF has been extensively studied in the hematopoietic system, little is known about its effects in solid tumors. We investigated the role of LIF in breast, kidney and prostate cancers. Using a clonogenic assay, we found that LIF significantly stimulated proliferation of 2 estrogen receptor‐positive breast cancer cell lines (MCF‐7 and T47‐D) in a dose‐dependent fashion at concentrations ranging from 10 to 200 ng/ml. This effect was observed both in the presence of FCS and under serum‐ and estrogen‐free culture conditions, suggesting that the effect of LIF is direct and does not depend on estrogen or any other cytokine. Neither line produced LIF protein, as assessed by ELISA. In contrast, the estrogen receptor‐negative breast cancer line MDA MB‐231 produced LIF but did not respond to either LIF or its neutralizing antibodies. Similarly, increasing concentrations of LIF did not affect the growth of primary kidney (A‐498), metastatic kidney (ACHN) and prostate (DU 145) cancer cell lines. These lines produce LIF, however, and antibodies to LIF significantly suppressed their proliferation, suggesting that they were maximally stimulated by the endogenously produced cytokine. Taken together, our data suggest that LIF acts as either a paracrine or an autocrine growth factor for breast, kidney and prostate cancers. © 1996 Wiley‐Liss, Inc.