Introduction Patients treated conservatively for acute diverticulitis (AD) are recommended colonic assessment to rule out an underlying Colorectal cancer (CRC). This has been questioned in patients with a CT diagnosis of uncomplicated AD. We analyzed the frequency of CRC and compared the characteristics of the CRC patients with controls. Method A cohort of patients treated conservatively for AD during 2005-2011 was identified through an administrative database. Patients diagnosed with CRC within 1 year after the index admission and four randomly selected controls were identified. The patients files were reviewed to verify the diagnosis and obtain information about the clinical characteristics and the management. A blinded review was performed of CT examinations. The expected number of CRC was calculated from age, sex, and period-specific incidence data. The characteristics of the CRC patients were compared with the controls in a nested case-control study. Results 890 patients (298 men and 592 women) were treated conservatively for AD. 12 patients were diagnosed with CRC within 1 year, and ten of them in the sigmoid, giving a Standardized Incidence Ratio of 20.0 (95 % CI 10.2-35.7, pamp;lt;0.001) for sigmoidal cancer. All CRC patients were aged over 70 years. The cancer was missed in six CT scans at the primary reading and in five at the blinded review. Conclusion Patients conservatively treated for AD are at high risk for an underlying CRC, especially if older than 70 years, which motivates routine follow-up. A CT diagnosis of uncomplicated AD does not rule out CRC.