We present the case of a 45-year-old man who underwent segmental resection for what was considered clinically to be a primary colonic carcinoma. Histopathologic evaluation revealed the tumor to be a malignant plasmacytoma of the colon. Additionally, the entire segment of distal ileum and colon was involved by an inflammatory process with the features of Crohn's disease, accompanied by focal glandular dyplasia of the colonic mucosa. This case reemphasizes that plasmacytoma of the colon can closely mimic carcinoma of the colon in its presentation. The association between plasmacytoma of the colon and Crohn's disease has not, to our knowledge, been described previously.