Salmonella colitis was encountered in eight patients. In seven, the disorder simulated ulcerative colitis both clinically and radiologically. The salmonella infection in the eighth patient was superimposed upon hitherto unrecognized ulcerative colitis. In mild cases the histological appearances of rectal biopsies were nonspecific, consisting of edema of the mucosa with focal inflammatory cell infiltration. More severe cases were characterized by neutrophils infiltrating the walls of degenerating crypts, and in one case there were microthrombi in the mucosa. One patient who was thought to have fulminant ulcerative colitis had a hemicolectomy. The resected specimen exhibited marked hemorrhage and ulceration. There were crypt abscesses in unulcerated areas but there was also extensive necrosis of the mucosa, hemorrhage in the mucosa and submucosa, and microthrombi extending from small vessels in the mucosa into venules in the submucosa similar to the picture seen in acute ischemic colitis. In this case there was intense edema and inflammation in the submucosa as well as in the mucosa.