Objective: The aim of this study was to elucidate the immunomodulatory effects that interferon α2a (IFNα2a) might exert in Behcet's disease (BD).Materials and Methods: Fourteen patients with active ocular BD were treated with human recombinant IFNα2a.Results: Compared with controls there was a significant elevation of monocytes and activated/regulatory T-cells.Naive T-cells were significantly lowered in patients.Under therapy, the number of NK-cells decreased significantly, whereas B-cells increased.The reduced level of expression of HLA-class-I-mols. on monocytes in HLA-B-51-pos. patients rose to levels comparable to HLA-B-51-neg. patients.Conclusion: These results implicate a participation of lymphocyte subpopulations in the pathogenesis of BD and may explain one mechanism by which IFNα2a exerts therapeutic effects.