Over 80% of colorectal cancers (CRCs) are sporadic/randomly arising tumors. Interval CRCs represent a subset of sporadic tumors that develop within 6-36 months after a negative colonoscopy. Interval CRCs are suggested to exhibit altered biological properties that contribute to rapid growth and proliferation. We hypothesize that chromosomal instability (CIN), or aberrant chromosome numbers, contributes to the etiology of Interval CRCs. We have assembled a Manitoban cohort of Interval and sporadic (control) CRC tumor samples, and established a fluorescence in situ hybridization approach to characterize CIN by enumerating specific chromosomes. The results of this study indicate that 75% of Interval CRCs exhibit a CIN phenotype, making CIN the most prevalent contributor to genomic instability in Interval CRCs. Only once we grasp a better understanding of the tumorigenic pathways through which Interval CRCs develop, can we tailor screening strategies and treatment options to specifically identify and combat this subset of sporadic CRC.