Leukemia inhibitory factor (LIF) is a pluripotent cytokine which has a diverse array of effects on hematopoietic and epithelial cells. Depending on the nature of the target cells, these effects can be growth-stimulatory or growth-inhibitory. Receptors for leukemia inhibitory factor (LIFR) have been identified on a variety of hematopoietic and epithelial cells. We have recently demonstrated in vitro growth stimulation of human breast cancer cells, both primary tumors and cultured cell lines, by LIF. To begin to understand the in vivo relevance of these observations, we investigated the expression of LIF and LIFR in human breast cancer specimens. Specimens from 50 cases were immunostained with mouse monoclonal antibodies D62.3 and M1 (to stain for LIF and LIFR, respectively). LIF expression was observed in 78% of the specimens and correlated with favorable biological features, i.e. low S-phase fraction (SPF) (P = 0.001) and diploidy (P = 0.08). LIFR expression was observed in 80% of the tumors and correlated with the presence of estrogen receptor (ER) (P = 0.04) and diploidy (P = 0.07). Coexpression of LIF and LIFR was associated with diploidy (P = 0.02) and low SPF (P = 0.05). LIF staining was primarily cytoplasmic whereas LIFR staining was cytoplasmic in the majority of cases and membranous in a minority of cases. The presence of LIFR in the primary tumor specimens correlated with the growth stimulation of tumor cells (derived from the same specimens) by exogenous LIF in methylcellulose colony assays. The findings support a widespread but probably complex role for LIF and LIFR in breast tumor growth regulation which should be investigated in greater detail in larger cohorts of tumors.