This review defines the entity pseudomembranous colitis and briefly outlines the supposed etiologic causes of pseudomembranous colitis including antibiotics. The incidence, mortality rate, and natural history of antibiotic-related pseudomembranous colitis is contrasted with other forms and causes of pseudomembranous colitis. The clinical spectrum of antibiotic-related pseudomembranous colitis, ranging from a nonbloody, watery diarrheal state to a life-threatening condition mimicking an acute surgical abdomen, is reviewed. The classic proctoscopic and pathologic findings, as well as common problems encountered in interpretation, are discussed. A complete review of the spectrum of radiographic findings is presented from the nonspecific to the quite characteristic radiographic findings, including both plain film and contrast studies of the colon. These findings are contrasted with the X-ray features of other inflammatory and ischemic colitides and a differential diagnosis is discussed. A section dealing with the treatment of antibiotic-associated pseudomembranous colitis will be included. This section will review the various modes of therapy that have been employed. Finally, a brief section will speculate on the possible etiologic role that antibiotics play in pseudomembranous colitis, including the alteration of the bacterial flora and possible effect on bile salt metabolism.