Nucleophosmin (NPM) is a ubiquitously expressed nucleolar phosphoprotein that continuously shuttles between the nucleus and cytoplasm(1). It has been proposed to function in ribosomal protein assembly and transport(2), and also as a molecular chaperone that prevents proteins from aggregating in the crowded environment of the nucleolus(3). The NPM gene is involved in several tumour-associated chromosome translocations, which have resulted in the formation of fusion proteins that retain the amino terminus of NPM, including NPM-ALK(4), NPM-RAR(5) and NPM-MLF1 (ref. 6). It is generally thought that the NPM component is not involved in the transforming potential of these fusion proteins(7), but instead provides a dimerization interface for the oligomerization and the oncogenic conversion of the various NPM partners (ALK, RAR, MLF1). Here we show that NPM interacts directly with the tumour suppressor p53, regulates the increase in stability and transcriptional activation of p53 after different types of stress, and induces p53-dependent premature senescence on overexpression in diploid fibroblasts. These findings indicate that NPM is a crucial regulator of p53 and suggest that alterations of the NPM function by NPM fusion proteins might lead to deregulation of p53 in tumours.