['The presence of advanced adenomas in younger individuals is a criterion for Lynch syndrome (LS). However, the utility of screening advanced adenomas for loss of mismatch repair (MMR) protein expression to identify suspected LS remains unclear.', 'Determine the prevalence of MMR defects to understand whether these patients harbor a defined genetic risk for CRC.', 'The study cohort included adult patients ≤45\xa0years of age with advanced adenomas (villous histology, ≥1\xa0cm in diameter, ≥3 polyps of any size) endoscopically removed between 2001 and 2011. Clinical records were reviewed along with detailed pathological review and immunohistochemical MMR analysis.', 'A total of 76 (40.1\xa0% male, age 40.6\xa0±\xa05.4\xa0years) patients met inclusion and exclusion criteria. Indications for colonoscopy were gastrointestinal (GI) bleeding 39 (51.3\xa0%), CRC in a first-degree relative 17 (22.4\xa0%) and somatic GI symptoms 20 (26.3\xa0%). Index colonoscopy revealed a median of 1 adenoma (range 1-4), mean diameter of 12.9\xa0±\xa07.1\xa0mm, 40 (52.6\xa0%) with villous histology. The mean follow-up duration was 3.3\xa0±\xa02\xa0years. Recurrent adenomas developed in 24 (31.6\xa0%), of which 8 (10.5\xa0%) were advanced adenomas; none of these patients developed CRC. One of 66 (1.5\xa0%) adenomas available for immunohistochemical (IHC) testing revealed loss of MLH1 and PMS2.'] Are advanced colorectal adenomas in patients under 45 years of age mostly sporadic?