Three cases of primary signet-ring carcinoma, or linitis plastica, of the colon are described. This rare form of colonic carcinoma possesses certain clinical, pathologic, and biologic characteristics that differ from those of the ordinary colorectal adenocarcinoma. Most importantly, the prognosis is poor, with only very few patients surviving more than one year following diagnosis. Some authors believe that the poor prognosis seen with this tumor may be due largely to a delay in diagnosis. Therefore, awareness of this form of colonic malignancy should be stressed so that early recognition may be made at a stage when curative treatment can still be instituted.