Pseudomembranous colitis (PMC) is a frequent cause of morbidity and mortality among hospitalized patients. Although diarrhea is the most common manifestation, PMC may be associated with intraperitoneal fluid accumulation in the severe cases. And a few cases showing both ascites and pleural effusion have been reported in patients with PMC. We report a case of PMC who showed elevated serum and ascites levels of carcinoembryonic antigen (CEA) with a normal CEA level in pleural effusion and who successfully recovered after oral administration of metronidazole. After treatment, the serum CEA level returned to the reference range.