Clostridium difficile colitis is diagnosed using an immunoassay or polymerase chain reaction (PCR) assay for toxins A/B. Since ultrasound is frequently used as a screening test for hospitalized patients suffering from different abdominal morbidities, we searched for sonographic indicators of C. difficile infection (CDI). In a prospective and blinded case-control study, abdominal ultrasound was performed on hospitalized patients for whom stool samples were sent for C. difficile toxin immunoassay. All patients with positive toxin were included as the case group and patients with negative toxin comprised the control group. Sonographic parameters of both groups were compared. Demographic variables of the 67 patients in the toxin-positive group were similar to those of the 71 patients in the toxin-negative group. The sonographic parameters which were found to be associated with CDI included colonic wall thickening, appearing in 61 (91%) patients of the toxin-positive group versus 15 (21%) patients of the toxin-negative group (p < 0.001), and also internal ring (24 versus 0%, p < 0.001), external ring (15 versus 0%, p < 0.001), ascites (24 versus 10%, p < 0.001), and diminution of large bowel content (16 versus 1%, p < 0.001). Bowel wall thickening had high positive and negative predictive values (0.80 and 0.90, respectively), while the other features had only high positive predictive values (0.7-1.0). Abdominal ultrasound may contribute to the diagnosis of C. difficile colitis in patients developing hospital-acquired diarrhea.