Objective: To investigate the association between diverticulitis and colon cancer in a large, nationwide cohort study. Background: Diverticulitis is a common disease, especially in the Western world. Previous articles have investigated the association between diverticulitis and colon cancer with inconclusive results. Methods: We conducted a population-based cohort study based on longitudinal Danish national registers with data from the period 1995 to 2012. Data were extracted from comprehensive Danish national registers containing information from both public and private hospitals. Patients with diverticulitis were identified from the registers and matched by sex and age (± 1 year) with a ratio of 1:10 to people who did not have a registration of diverticulitis or diverticulosis. Main outcome was the event of colon cancer. Subgroup analyses were performed to investigate the effect of colonoscopies and treatment on the colon cancer rate after diverticulitis. Results: A total of 445,456 people were included, of whom 40,496 had a diagnosis of diverticulitis. The incidence of colon cancer in the group with diverticulitis (4.3%) and the group without diverticulitis (2.3%) differed significantly (P < 0.001) with an incidence rate ratio of 1.86 (95% confidence interval, CI, 1.77-1.96). When adjusted for possible confounders, the association between diverticulitis and cancer remained significant with an odds ratio (OR) of 2.20 (95% CI 2.08-2.32) (P < 0.001). Those with diverticulitis, who had no colonoscopy, had an increased risk of colon cancer compared with those without both diverticulitis and colonoscopy with an OR of 2.72 (95% CI 2.64-2.94) (P < 0.001). Conclusions: We found a strong association between development of diverticulitis and colon cancer. This raises several questions regarding the possible causal association and warrants further studies. Patients with diverticulitis should undergo endoscopic surveillance for colon cancer.