Phenotypic characterization and distribution of gamma/delta T lymphocytes in the intestinal mucosa were investigated in ulcerative colitis and Crohn's disease by immunohistochemistry. The ratio of delta(+) cells to CD3(+) cells in the intraepithelial space of colon was decreased in Crohn's disease (13%) and strikingly decreased in ulcerative colitis (8%) compared with the control (36%). Delta(+) cells in the lamina propria were also decreased, particularly in the distal ileum of Crohn's disease (4%), compared with the control (15%). On the contrary, the cells gathered at the severe inflammatory sites with other inflammatory cells, including beta(+) cells, and were densely distributed in the T-cell zone around lymphoid follicles. Phenotypic characterization showed that delta(+) lamina proprial lymphocytes of colon were mainly CD4(-)CD8(-) in the control (80%) and Crohn's disease (59%). However, in ulcerative colitis, CD4(-)CD8(-) delta(+) lymphocytes were rarely found (3%). This reflects the difference of immunologic background between the two diseases. Immunoelectron microscopically, these cells in inflammatory bowel disease were rich with vesicular structures in cytoplasms, whereas those in the control group contained electron-opaque granules. The decrease and the morphological change may be closely related to the weakness of mucosal defense.