We present a girl who developed adrenocortical adenoma at the age of 1 year and osteosarcoma at the age of 5 years. There was no history of cancer in her parents and their relatives. However, both tumors were typical for the Li–Fraumeni syndrome (LFS), and the patient met criteria for germline TP53 mutation testing. A mutation in codon 282 (Arg282Trp) was identified in her blood lymphocyte genomic DNA. The substitution was found in neither of her parents, which indicated a possibility of a de novo mutation. Unexpectedly, sequencing of the DNA of the patient repeatedly showed allelic imbalance in favor of the normal allele. This observation prompted us to investigate the putative somatic mosaicism in the patient consisting of normal cells and cells heterozygous for the mutation. The imbalance was also examined in two other non-invasively sampled tissues, buccal cells, and cells from the urine sediment, and sequencing was confirmed with two other independent methods. While the findings in blood and the urine sediment were similar, in buccal cells both alleles were present in equal amounts. The allele ratio in lymphocytes was consistent with a mosaic where about 2/3 of cells carried two normal alleles and only 1/3 was heterozygous for the mutation. Despite the mosaicism the girl developed two early childhood tumors of mesodermal origin, and her phenotype was thus not milder than that of other germline TP53 mutation carriers. To our knowledge this is the first description of somatic mosaicism for a de novo TP53 mutation in LFS. © 2008 Wiley-Liss, Inc.