Gamma-interferon (IFN-γ) is detected at a higher level in the sera of patients with Behçet’s disease than in control subjects. To investigate the cellular source of IFN-γ, peripheral blood lymphocytes (PBL) from patients were cultured and found to produce IFN-γ spontaneously in their fresh culture fluids. The average IFN titers in cultures of the patients’ PBL were significantly higher than those of the control cultures (p < 0.0001). The IFN activity was significantly higher in cultures of the patients in the ocular convalescent stage than in the exacerbation stage (p < 0.0001), as was the case with serum interferon activity. Fractionation of PBL showed that only Tγ lymphocytes of the patients produced IFN-γ spontaneously in the cultures. No IFN was produced in the culture fluids of T non-γ cells, non-T cells or macrophages. Tγ cells and IFN-γ may therefore play a significant role in the immunopathophysiology of Behçet’s disease.