OBJECTIVE Routine diagnostic work revealed cell aggregates reminiscent of lymph nodes in the bowel submucosa in occasional cases of chronic inflammatory bowel disease. We therefore investigated whether they fulfill criteria for classification as lymph nodes. METHODS Colon with terminal ileum from a patient with florid Crohn's disease and a colectomy specimen from a patient with ulcerative colitis were investigated. Sections were immunostained with antibodies that recognize endothelial and sinus-lining cells, immune-accessory cells, and lymphoid cells. RESULTS Circumscribed collections of cells that fulfill all the major criteria for classification as lymph nodes were found in the large and small bowel. They had marginal and intermediate sinuses (positive for BMA 120, CD34, CD31, X-11, and von Willebrand's factor), afferent lymph vessels, T- and B-regions, and a capsule. Small collections composed predominantly of B cells that had only a marginal sinus were also occasionally observed. CONCLUSIONS Secondary mucosa-associated lymphoid tissue, typically seen as follicular lymphoid hyperplasia, also appears to occur as secondary submucosal lymph nodes. This phenomenon seems inconsistent with the notion that lymph nodes do not develop after birth. We have also noted secondary development of lymph nodes in lymphangioma and lymphangioleiomyomatosis. It is possible that local lymph vessel proliferation, possibly with chronic lymphedema of the tissue involved, is an important prerequisite for lymph node neogenesis.