A case of acute perforation of a diverticulum of the ascending colon is presented. The ultrasound features that helped in making a pre-operative diagnosis are discussed. This rare surgical emergency and the similar clinical entities of perforated caecal or transverse colon diverticula are often clinically misdiagnosed as more common conditions such as acute appendicitis, acute cholecystitis or perforated ulcer. Awareness of this entity will help radiologists in making a correct pre-operative diagnosis.