Purpose: The aim of the study was to investigate short-term mortality, readmission, and recurrence in a national cohort of patients with Hinchey Ib-II diverticulitis. Methods: The retrospective cohort-investigation was conducted using a database consisting of the entire Danish population (n = 6,641,672) in year 2000–2012, formed by linking the Danish Registers. Patients admitted with acute Hinchey Ib-II diverticulitis were identified from ICD-10 discharge codes and stratified according to treatment into an operative, drainage, and antibiotics group. The primary outcome was 30-day mortality from admission, secondary outcomes were mortality, readmission, and recurrence within 30 days post-discharge. The study was reported using RECORD guidelines. Results: A total of 3148 eligible patients were identified. The cohort had a mean age of 65.1 year, 25.6 % had previously been admitted with diverticulitis, and 48.1 % had registered comorbidities. Within 30 days from admission, 8.7 % of the patients died. Of patients discharged, 2.5 % died, 23.8 % was readmitted, and 5.9 % was readmitted due to diverticulitis within 30 days from discharge. In multivariate analyses, increasing age was associated with mortality at odds-ratio (95 % CI) 1.10 (1.09–1.12). Previous complicated and uncomplicated diverticulitis reduced mortality with odds-ratio 0.50 (0.33–0.76) and 0.73 (0.58–0.92), while uncomplicated diverticulitis also increased risk of recurrence with odds-ratio 1.51 (1.24–1.84). Glucocorticoid usage was associated with mortality with odds-ratio 1.49 (1.23–1.81) and readmission with odds-ratio 2.91 (1.24–6.80). Conclusion: Acute diverticulitis with abscess formation is a severe and life-threatening condition. Direct comparisons of treatment groups were not possible due to possible confounding by indication.