Objective To determine the risk of developing complicated colonic diverticular disease (CCDD) with prior episodes of acute diverticulitis and determine the mortality of the spectrum of CCDD.

Design Population-based cohort study.

Setting Computerised records from the General Practice Research Database linked to Hospital Episode Statistics data from the UK.

Participants Patients and controls registered in the General Practice Research Database from 1990 to 2007.

Main outcome measures Mortality was calculated and Cox regression modelling used to provide adjusted HRs and 95% CI. Logistic regression was used to model the effect of prior acute diverticulitis on the development of complications.

Results 2950 patients (1872 (63.5%) female) had a diagnosis of CCDD (8739 controls). A total of 1042 (35.3%) patients died compared with 2062 (23.6%) controls. Most excess deaths occurred in the first year after the complication. Patients with a perforation/abscess had a 4.5-fold increase in 1-year mortality (HR 4.55, 95% CI 3.74 to 5.52) compared with the general population, whereas those with a fistula or stricture had a 2.5-fold increase in mortality (fistula HR 2.60, 95% CI 1.47 to 4.62; stricture HR 2.41, 95% CI 1.86 to 3.11). Although most patients (2133 (72.3%)) had suffered no prior episodes of acute diverticulitis, increasing episodes of acute diverticulitis were associated with an increased risk of developing a fistula (two or more prior episodes, OR 1.54 95%, CI 1.08 to 2.19), but there was no clear relationship with stricture or perforation/abscess.

Conclusions Although most patients have experienced no prior episodes of acute diverticulitis, fistula formation is preceded by bouts of inflammation. Excess 1-year mortality across the spectrum of CCDD compared with the general population is substantial.