T-lymphocyte subpopulations were measured in the blood of 20 patients with ulcerative colitis (11 in clinical remission and 9 in exacerbation) and 9 healthy persons, before and after in vitro incubation with dexamethasone. Although we found no significant differences in the helper/suppressor T cell ratio between patients and controls before in vitro incubation, addition of dexamethasone in culture induced a significant decrease of the ratio only in patients with ulcerative colitis in exacerbation. The percentage of natural killer cells before and after incubation with dexamethasone was not altered.