Purpose: Lynch syndrome accounts for 2-4% of all colorectal cancer, and is mainly caused by germline mutations in the DNA mismatch repair genes. Our aim was to characterize the genetic mutation responsible for Lynch syndrome in an extensive Colombian family and to study its prevalence in Antioquia. Methods: A Lynch syndrome family fulfilling Amsterdam criteria II was studied by immunohistochemistry and by multiplex ligation-dependent probe amplification (MLPA). Results were confirmed by additional independent MLPA, Southern blotting, and sequencing. Results: Index case tumor immunohistochemistry results were MLH1-, MSH2+, MSH6+, and PMS2-. MLPA analysis detected a duplication of exons 12 and 13 of MLH1. This mutation was confirmed and characterized precisely to span 4219 base pairs. Duplication screening in this family led to the identification of six additional carriers and 13 noncarriers. We also screened 123 early-onset independent colorectal cancer cases from the same area and identified an additional unrelated carrier. Conclusion: A novel duplication of exons 12 and 13 of the MLH1 gene was detected in two independent Lynch syndrome families from Colombia. A putative founder effect and pre-screening Lynch syndrome Antioquia families for this specific mutation before thorough mismatch repair mutational screening could be suggested. Genet Med 2011:13(2):155-160.