We report a constitutional point mutation of codon 278 in exon 8 of the TP53 gene that has not yet been described as a germ-line mutation. A 52-year-old female developed multiple primary malignancies (liposarcoma, breast cancer, malignant histiocytoma, occult adenocarcinoma). The mutation found in her tumour and peripheral blood lymphocyte DNA is a cytosine to thymine transition at the second position of codon 278 resulting in an amino acid exchange from proline to leucine in the DNA-binding domain. Evaluation of the patient's family revealed that both of her sons were affected by the same mutation. Although the patient's mother had died already, we were able to demonstrate by polymorphic microsatellite analysis that the defective allele originated from the maternal side. As four brothers and one sister had inherited the same allele, which however was wild type, we were able to show that the mutation must have occurred in the germ cells of the patient's mother and that it may therefore be called de novo. This explains the lack of a high cancer incidence in the family history. All tumours tested showed positive immunohistochemical staining for p53. Loss of heterozygosity was found in five of seven tumours, one showing chromosome 17 monosomy.