Interleukin-7 (IL-7), a haematopoietic growth factor, is known to induce the differentiation and proliferation of some haematological malignancies including certain types of leukaemias and lymphomas. However, little is known about its role in solid tumours, including breast cancer. In this study, the expression level of IL-7, IL-7 receptor (IL-7R) and their downstream signalling molecules, including the Janus kinases (Jak-1 and Jak-3), phosphoinositide 3-kinase (PI3-K) and signal transducers and activators of transcription (Stat-5) were analysed using the reverse transcriptase-polymerase chain reaction (RT-PCR), real-time quantitative PCR and immunohistochemistry in a cohort of patients with breast cancer. The results were analysed in relation to tumour grade, TNM stage, patients' prognosis (using the Nottingham Prognostic Index (NPI)) and survival. The levels of expression of IL-7, IL-7R, Jak-1, Jak-3, PI3-K and Stat-5 were significantly higher in the most aggressive tumours. With the exception of Stat-5 expression, the transcript copies of IL-7 and all other signalling molecules were higher in patients with the worst prognoses (NPI3) and in patients who died from breast cancer after 72 months of follow-up. This aberrant expression of IL-7 and its signalling intermediates in invasive breast cancers could have significant diagnostic and prognostic implications. Measuring these molecules in breast cancer tissues may provide, for the first time, important molecular indicators of tumour differentiation, aggressiveness, nodal status, prognosis and patient survival.