The p53 and MDM2 genes are part of a physiological pathway frequently impaired in human cancer. With the exception of tumours occasionally associated with hereditary predisposition, childhood malignancies have not been studied in detail yet. This is the first report on the analysis of p53 and MDM2 in a group of non-hereditary paediatric neoplasms referred to as the Ewing tumours (ETs). Thirty-seven primary tumours and cell lines from 19 patients were screened for the presence of p53 mutations. Only 5% of the primary tumour specimens were found to carry an alteration within this gene. However, p53 mutations were 10-fold enriched in ET cell lines, thus indicating a selective growth advantage in vitro. Strikingly, five out of nine alterations detected were missense mutations within codon 273, which were previously reported to impair only partially the normal p53 function. Two single-base substitutions occurred at codons 277 and 176, and two alterations were loss-of-function mutations. Investigation of the MDM2 gene revealed neither gene amplification in the primary tumours and cell lines nor significant overexpression in any of the cell lines. Our data therefore suggest that impairment of cellular mechanisms involving p53 is rare in a distinct group of childhood malignancies.