Genetic mosaicism is the presence of genetically different cell populations within an individual and can be associated with a milder disease phenotype. We describe a somatic mosaicism in a Lynch syndrome patient with a MLH1 gene mutation (c.1050delA). Since she was the sister of a heterozygous proposita, the mosaicism appeared to be caused by reversion of an inherited mutation and not a de novo mutation. In order to better understand her cancer risk, we tested different tissues to quantify the amount of mutated allele in several districts. The mosaicism was analyzed using DNA sequencing, primer extension, and dHPLC. The MLH1 mutation was present in somatic cells representative of the three embryonic layers and its percentage was ≥80% in both blood and tissues. Since this patient had a relevant quota of mutated cells, a significantly milder phenotype is not expected. © 2009 Wiley-Liss, Inc.