Hereditary non-polyposis colorectal cancer (HNPCC) is an autosomal dominant syndrome characterized by a predisposition to early-onset colorectal, endometrial and other cancers. The tumours typically exhibit microsatellite instability due to defective mismatch repair. HNPCC is classically caused by heterozygous loss-of-function mutations within the mismatch repair genes MLH1, MSH2, MSH6 and PMS2, but no pathogenic mutations are identified in a third of cases. In recent years, constitutional epimutations of the MLH1 gene, characterized by soma-wide allele-specific promoter methylation and transcriptional silencing, have been identified in a handful of mutation-negative HNPCC cases. In contrast to genetic mutations, MLH1 epimutations are reversible between generations and thus display non-Mendelian inheritance. This review focuses on the aetiological role of constitutional MLH1 epimutations in the development of HNPCC-related cancers. The molecular characteristics, clinical ramifications and potential mechanism underlying this defect are discussed. Recommendations for the selection of cases warranting screening for MLH1 epimutations are proffered.