Samples of peripheral blood from 26 patients with Crohn's disease (CD) and 26 healthy age- and sex-matched controls were tested simultaneously for B and T lymphocytes and T-lymphocyte subpopulations with receptors for IgM (TM) and IgG (TG). Patients with CD had reduced proportions of T lymphocytes, and this reduction showed a significant correlation to the CD activity index (r = -0.65, p less than 0.01). There was slight reduction of TM only (p less than 0.05) inpatients with highly active disease but not in the total population of patients studied. Proportions of B and TG cells were similar in patients and controls. Patients with no clinical or radiological but histological signs of active disease had T lymphocytes and subpopulations like patients with inactive disease. This suggests that the reduction of T cells and T-cell subpopulations in CD are secondary effects. With regard to T-lymphocyte subpopulations, our results are in contrast to a recently published report and do not suggest that analysis of T cells according to the expression of Fc receptors helps in the understanding of functional changes in the T-cell system in patients with CD.