X-linked agammaglobulinemia (XLA) is a primary immunodeficiency disorder caused by germline mutation of the Bruton tyrosine kinase (BTK) gene. Most of the patients diagnosed X-LA suffer from recurrent infections of the respiratory and gastrointestinal tracts. Increased risk of malignancy in X-LA patients include lymphoma, gastric cancer, colorectal cancer. We report a case of 32-years-old male patient with X-linked agammagolbulinemia and rectal cancer. Agammaglobulinemia was diagnosed at 13 years old. He underwent colonoscopy for hematochezia. An ulceroinfiltrative mass was found during colonscopy and biopsy revealed moderately differentiated adenocarcinoma. Subsequently, he underwent a anterior resection. (Korean J Gastrointest Endosc 2010;41:360-363)