A normal colonoscopy two years ago can be cancerous two years later. A colonoscopy is a way to check for lesions in the rectum and colon. Cancer refers to certain benign tumors or lesions that progressively develop into cancer. Tumors grow very fast. It is possible that two years ago the cancer cells were very small and had not broken through the mucosa, so the colonoscopy was normal, but it is possible that after two years, the cancer cells have grown exponentially and have infiltrated and metastasized, so cancerous lesions will appear after two years. The time from benign to malignant bowel cancer can take upwards of ten years in some patients, or as little as a few months in others. It should be noted that cancerous patients can reduce the adverse effects of the disease through early detection, diagnosis and treatment, and many patients are likely to survive for a long period of time, so patients are advised to build up confidence in treatment.