Lower gastrointestinal (GI) bleeding is a well-known symptom of colorectal cancer (CRC). Whether incident GI bleeding is also a marker of other GI cancers remains unclear.This nationwide cohort study examined the risk of various GI cancer types in patients with lower GI bleeding. We used Danish medical registries to identify all patients with a first-time hospital diagnosis of lower GI bleeding during 1995-2011 and followed them for 10 years to identify subsequent GI cancer diagnoses. We computed absolute risks of cancer, treating death as a competing risk, and calculated standardized incidence ratios (SIRs) by comparing observed cancer cases with expected cancer incidence rates in the general population.Among 58,593 patients with lower GI bleeding, we observed 2,806 GI cancers during complete 10-year follow-up. During the first year of follow-up, the absolute GI cancer risk was 3.6%, and the SIR of any GI cancer was 16.3 (95% confidence interval (CI): 15.6-17.0). Colorectal cancers accounted for the majority of diagnoses, but risks of all GI cancers were increased. During 1-5 years of follow-up, the SIR of any GI cancer declined to 1.36 (95% CI: 1.25-1.49), but risks remained increased for several GI cancers. Beyond 5 years of follow-up, the overall GI cancer risk was close to unity, with reduced risk of rectal cancer and increased risk of liver and pancreatic cancers.A hospital-based diagnosis of lower GI bleeding is a strong clinical marker of prevalent GI cancer, particularly CRC. It also predicts an increased risk of any GI cancer beyond 1 year of follow-up.