Intestinal lamina proprial lymphocytes were enumerated in 13 patients with Crohn's disease (CD), 7 with ulcerative colitis (UC) and 9 control subjects with grossly normal bowel. The percentages and absolute numbers of B cells and complement receptor-bearing lymphocytes in the patients with UC did not differ significantly from those of the control group; but in CD, the percentages of these cells were significantly decreased despite that the absolute numbers were normal. The percentages and absolute numbers of T cells identified by E rosettes (regular and stable) were normal in both CD and UC. In both diseases, significantly increased percentages but normal absolute numbers of mature T cells were identified by monoclonal antibody staining with the CD3 reagent. There was an increased proportion of suppressor/cytotoxic T cells and a concomitant significant decrease of the helper to suppressor T-cell ratio (using either percentages or absolute numbers of cells) in tissues from patients with CD, but not with UC as compared with the control group. These results suggest that the imbalance of T-cell subpopulations in the intestinal lamina propria may play a role in the pathogenesis of CD.