The importance of nucleolar changes in cancer cells is underestimated in tumour pathology. There is evidence that the nucleolus is the mirror of a series of metabolic changes that characterize cancer cells. Cell entry into the cell cycle is always associated with up-regulation of the nucleolar function and increased nucleolar size, which are also directly dependent on the rapidity of cell cycle progression. Furthermore, alterations of the major tumour suppressor retinoblastoma (Rb) and p53 pathways also contribute to the stimulation of nucleolar function and to nucleolar enlargement. High cell growth fraction, high cell growth rate and disruption of the Rb and p53 pathways are responsible for greater aggressiveness of cancer tissues. Therefore, the evaluation of nucleolar size allows one to obtain reliable information on the clinical outcome of the cancer: the greater the nucleolar size, the worse the tumour prognosis. Indeed, a series of studies carried out on numerous human tumours has shown that nucleolar hypertrophy (prominent nucleolus) was an independent predictive and prognostic parameter of a fatal clinical outcome.