Purinergic signal transduction mechanisms have been appreciated as a complex intercellular signalling network that plays an important regulatory role in both short- and long-term processes in practically every living cell. One of the most intriguing aspects of the field is the participation of ATP and other purine nucleotides in the determination of cell fate and the way they direct cells towards proliferation, differentiation or apoptosis, thereby possibly taking part in promoting or preventing malignant transformation. In this review, following a very brief introduction to the historical aspects of purinergic signalling and a concise overview of the structure of and signal transduction pathways coupled to P2 purinergic receptors, the current theories concerning the possible ways how extracellular ATP can alter the function of tumour cells and the effectiveness of anticancer therapies are discussed, including pharmacological, nutritional, vasoactive and 'anti-antioxidant' actions of the nucleotide. The effects of ATP on animals inoculated with human tumours and on patients with cancer are looked over next, and then an overview of the literature regarding the expression and presumed functions of P2 purinoceptors on tumour cells in vitro is presented, sorted out according to the relevant special clinical fields. The article is closed by reviewing the latest developments in the diagnostic use of P2 purinergic receptors as tumour markers and prognostic factors, while discussing some of the difficulties and pitfalls of the therapeutic use of ATP analogues.