Four cases of pseudomembranous colitis following antibiotic therapy are reported. Special attention is focused on clinical manifestations and histologic features. The clinical manifestation along with history is quite characteristic, if not specific, suggesting the colonic lesions to be post-antibiotic. Colonoscopic examination is diagnostic in four cases studied, and the lesions consist of discrete 0.2 to 2cm raised plaques adherent to an edematous, friable mucosa. The pathologic picture in pseudomembranous colitis is quite distinctive. The diagnostic histological findings include a "mushroom" or "volcano" appearance of the pseudomembrane with sudden transition to normal mucosa adjacent to the lesion.