On rare occasions, secondary Epstein-Barr virus (EBV)-associated B cell lymphoma can develop in a patient with angioimmunoblastic T-cell lymphoma (AITL). We report a case of a 66-year-old Japanese woman who developed diffuse large B-cell lymphoma (DLBCL) in her small intestine after chemotherapy for AITL. She was found to have panperitonitis due to perforation of the small intestine. Partial ileectomy specimen showed DLBCL cells infiltrating into the intestinal wall. In situ hybridization for EBV-encoded RNA revealed positivity in the lymphoma cells. The lymph nodes diagnosed as AITL were negative for EBV infection and there was no coexistence of B cell neoplasms in them. We thought small bowel perforation in this case was caused by EBV-associated B cell lymphoma secondary to AITL. Our case showed a remarkable deficiency of cellular immunity after chemotherapy, which we postulate was related to the cause of occurrence of B-cell lymphoma.