Signet-ring cells associated with pseudomembranous colitis.

After being treated for pneumonia with antibiotics, an 82-year-old man with a history of colonic carcinoma developed pseudomembranous colitis with toxic megacolon. In addition to the classic features of pseudomembranous colitis, the colon displayed signet-ring cells confined to the surface epithelium and crypts. A misdiagnosis of recurrent carcinoma was avoided after a review of the history and the endoscopic findings and by noting the location of the signet-ring cells.