Colorectal cancer (CRC) surveillance guidelines are designed to decrease the incidence of cases. However, small subgroups of CRC are discovered after a screening examination in which no suspicious lesions were detected. Post colonoscopy CRC is thought to be multifactorial due to technical factors or distinct tumor biology playing an important role in pathogenesis. A 58 year old female presented with constipation, hematochezia and weight loss. No past medical history, & family histroy was non-contributory. She underwent a screening colonoscopy revealing internal hemorrhoids. One year later, patient had worsening symptoms, CT abdomen showed transverse colon mural thickening with abrupt luminal narrowing. Repeat colonoscopy revealed a 3-5 cm stricture in the transverse colon with biopsy consistent with hyperplastic changes. Given the high suspicion of malignancy, repeat biopsy showed moderately differentiated invasive adenocarcinoma, and microsatellite instability (MSI) negativity. Repeat CT for staging at 3 months revealed omental metastasis & partial bowel obstruction. Patient underwent exploratory laparotomy which revealed diffuse carcinomatosis throughout the abdominal wall, small bowel, & large lesion in the transverse colon. Patient underwent palliative colonic stent placement and chemotherapy. Interval colorectal carcinoma (iCRC) is defined as CRC appearing after a negative screening exam, & before the date of the next recommended screening. A recent meta-analysis showed a prevalence of 3.7% of cases if ICRC. Risk factors for iCRC include tumor biology, technical factors, nonpolypoid colorectal neoplasms, serrated lesions, and hereditary cancer syndromes. iCRC most commonly seen in the proximal colon, have mucinous histology, MSI positivity, & are associated with IBD and diverticular disease. Interestingly, iCRC has a 37% lower risk of mortality when compared to detected CRC. They also have similar one year survival rate compared to those who develop CRC after 10 years from last colonoscopy. Our case is unique since our patient developed a rapidly progressive iCRC despite no family history, MSI negativity, and compliant with the colonoscopy surveillance guidelines. Current colonoscopy surveillance guidelines may be inadequate to prevent such cases of interval colorectal cancers. Further research is needed to determine whether these interval colorectal cancers arise as a result of missed lesions or accelerated neoplastic progression.1577_A Figure 1. Neoplastic cells in cords and glandular pattern, at right lower corner.1577_B Figure 2. Neoplastic glands consists of markedly atypical cells with nuclear pleomorphism and nuclear hyperchromasia.1577_C Figure 3. Transverse colon stricture