A 13 year old boy with Crohn's disease had a prominent expansion (over 40%) of gamma delta T cell receptor-positive cells (gamma delta T cells) in the peripheral blood. The gamma delta T cells were gradually decreased from 48 to 22% in 18 months. Anti-gamma delta T cell receptor-triggered cytotoxicity was evident at onset and after 1 year. The significance of peripheral expansion of gamma delta T cells in a patient with Crohn's disease is discussed.