Background: Acute colonic diverticulitis is common in the Western world representing a growing burden on health care. We aimed to report the factual epidemiological and demographic characteristics in patients with acute diverticulitis in a large nationwide population. Method: We conducted a population-based cohort study from 2000 to 2012 on the complete Danish population, which included all patients with acute colonic diverticulitis. Data were composed through two national longitudinal registries. The study main outcomes were demographic development regarding hospital admission, age, gender, geographical residency, and seasonal information. Results: A total of 101,963 acute hospital contacts were identified from 2000 to 2012, of these 44,160 were due to acute diverticulitis. From 2000 to 2012, overall admission rates for complicated diverticulitis increased significantly with 42.7%. There was a small increase in hospital admissions due to acute diverticulitis, and uncomplicated diverticulitis accounted for 83–88% of all admissions. No significant development was seen in cases of uncomplicated diverticulitis. The majority of patients were older than 50 years (85%) and 60% were women. The male gender dominated in patients younger than 50 years (58%), whereas women dominated above 50 years (63%). Mean age and dominating age group decreased significantly from 2000 to 2012 for both genders. A significantly larger proportion of male patients had complicated diverticulitis than uncomplicated diverticulitis. Most admissions were seen during autumn. Conclusion: We found that acute colonic diverticulitis has been progressing over the last decade with more severe cases of disease. Our findings underline the need for further research to identify the relevant risk factors and causal circumstances.