Quantitative analysis and immunohistochemical studies of small intestinal mucosa were performed to investigate the mechanism of mucosal damage in 10 patients with food-sensitive enteropathy. Jejunal biopsy specimens were taken before and after treatment and after clinical relapse following a challenge test. The low villous height of untreated patients normalized after introduction of an elimination diet but declined again to subnormal level after a challenge test. Several other types of cells were significantly increased in the untreated patients in comparison to controls. These included HLA-DR+ (DR+) CD4+ cells in the lamina propria and intraepithelial CD8+ cells. Moreover, those cell patterns, such as increased DR+ CD4+ cells and CD8+ cells, normalized with treatment but regressed to pretreatment levels when the patients were challenged. These findings suggest that activated CD4+ cells in the lamina propria of the small intestinal mucosa, probably by releasing cytokines, may play an important role in contributing to mucosal damage in patients with food-sensitive enteropathy.