Current literature provides significant evidence to support the concept that there are limited subpopulations of cells within a solid tumour that have increased tumour-initiating potential relative to the total tumour population. Such tumour-initiating cells have been identified in leukaemia and in a variety of solid tumours using different combinations of cell surface markers, suggesting that a tumour-initiating cell heterogeneity exists for each specific tumour. These studies have been extended to endometrial cancer; and herein we present several experimental approaches, both in vitro and in vivo, that can be used to determine whether such populations exist, and if so, to characterize them. These methods are adaptable to the investigation of tumour-initiating cells from other tumour types.