Acute colitis is an acute condition of the colon. For the radiologist, it is mainly diagnosed during differential diagnosis of acute abdominal conditions. There are many causes of colitis and the degree of its severity varies. A CT scan is the best imaging examination for diagnosing it and also for analysing and characterising colitis. The topography, type of lesion and associated factors can often suggest a precise diagnosis but it is nevertheless essential to integrate these findings into the clinical context and take laboratory values into account. The use of endoscopy is still the rule where a doubt remains, or to obtain necessary histological evidence.