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Background: Familial colorectal cancer type X (FCCX) is a subtype of mismatchrepair (MMR)-proficient colorectal cancerin whichthe patients are clinicallyat risk for Lynch syndrome (LS), a common hereditary cancer predisposing syndrome.In this study, we describeda new clinicopathological feature of the condition in central Iran. Subjects and Methods: We designed a descriptive, retrospective study to screenat-riskcolorectal cancer (CRC) patients,usingAmsterdam II criteria and Molecular analysis in Isfahan (central Iran) throughout 2000-2013 period. Results: 219 early-onset (≤ 50 years) CRC patients of 1659 were selected for the evaluation. Amsterdam II criteria were positive in 45 families; of whom 31 were finally analyzed by molecular testing.  MMR deficiency was detected in 7/31 probands (22.6%) as affected to LS, so 24 families (77.4%) were identified as FCCX. The mean age of the probands at diagnosis among FCCX families was 45.3 years (range 24-69) versus 38.0 years (range 31-50) in LS families.The frequency of CRC among FCCX and LS families was calculated 27.9% and 67.5%, respectively. Also, the most frequent extracolonic cancer among both FCCX and LS families was stomach by 25.5% and 30.8%, respectively. Tumor site was proximal to the splenic flexure in 20.8% and 57.1% of index CRC patients in FCCX and LS families, respectively. Conclusion: Given the relative high frequency of FCCXand its different phenotype among Iranian populations, we need to set up more advanced molecular studies for exploration of unknown molecular pathways leading to tumorigenesis in this class of CRC patients.