The p53 gene located in the short arm of chromosome 17 at position 17p13, is involved in the negative regulation of cellular growth. p53 mutation seems to be the most frequent genetic alteration found in human cancer. Mutant conformation of the p53 gene is associated with cell proliferation and tumour progression, and in most cases implies p53 stabilization, which renders the p53 protein detectable through the use of immunohistochemical techniques. p53 expression is a frequent finding in high grade lymphomas of either B or T cell lineage, having been detected in 30% of cases in our series. The focal presence of p53+ cells was seen in a wide range of low and high grade lymphomas, including lymphadenitis and reactive tonsils. In 37.5% of cases this increased expression of p53 was secondary to mutation in highly conserved regions (exons 5-8). Unlike findings reported in other tumours, in lymphomas, p53 expression seems to be secondary to genetic alterations other than p53 mutation. Initial data suggest that the MDM2 protein could be involved in inactivating p53 protein in most of these cases. Finally, p53 expression has been found to be a poor prognostic marker in high grade B-cell lymphomas in a large series of cases. High p53 expression was associated with a short survival, this relation being stronger in cases with simultaneous bcl2 expression.