T cells and T cell subsets were studied in the peripheral blood of 18 patients with Behcet disease (8 active and 10 inactive) and 20 healthy controls. When the active disease group was compared with the inactive disease group and the control group, there was a significant increase (85.7% versus 71.3% and 69%) in the mean percentage of T cells for the active disease group. The mean percentage of CD4 cells for the active disease group (36%) was also significantly decreased from the mean value of the inactive disease group (36%) was also significantly decreased from the mean value of the inactive disease group (41%) and the controls (40%). There was a concomintant significant increase of CD8 cells of the group with active disease (43%) over the mean value for the group of inactive disease (32%) and controls (33%). These results indicated that there were alterations of T cells and T cell subsets in Behcet disease. In contrast to other studies, we found that the T cell number increased with disease activity in our patients; but the significance of single measurement of T cell number increased with disease activity in our patients; but the significance of single measurement of T cell subsets in a course of chronic disease must be taken into consideration.