p27 KIP1 is a cyclin dependent kinase inhibitor, which may act as a potential suppressor gene. Several lines of evidence support the hypothesis that reduced p27 KIP1 expression is related to uncontrolled cell proliferation and tumorigenesis. Low immunohistochemical expression of p27 KIP1 in human neoplasm seems related to tumor progression and poor prognosis. In breast cancer, low p27 is associated with high tumour grade and loss of oestrogen receptor, and it has been suggested that low p27 KIP1 is a powerful and independent prognostic marker of poor clinical outcome. There are however some discrepant results: a few studies, some of which conducted on large series of patients, do not support an independent role of p27 KIP1 as a prognostic marker. We are indeed faced with an intriguing hypothesis, but many more studies are needed to evaluate the real value of p27 KIP1 as a prognostic marker.