Rationale: Occupational exposure to beryllium (Be) can result in chronic granulomatous inflammation characterized by the presence of Be-specific CD4⁺ T cells. Studies show that oxidative stress plays a role in the pathogenesis of chronic inflammatory disorders. Objectives: We hypothesized that Be-induced oxidative stress modulates the proliferation of Be-specific CD4⁺ T cells. Methods: Thirty-three subjects with chronic beryllium disease (CBD), 15 subjects with beryllium-sensitization (BeS) and 28 healthy normal control subjects were consecutively enrolled from the National Jewish Medical and Research Center Occupational and Environmental Health Clinic. Measurements and Main Results: All studies were performed using Ficoll-Hypaque isolated peripheral blood mononuclear cells (PBMC) from subsets of the study subjects. Decreased intracellular levels of the thiol antioxidants, glutathione and cysteine, were observed in PBMCs from BeS and CBD subjects, as compared to healthy controls. Be-stimulation decreased intracellular thiol antioxidants by > 40%, accompanied by increased ROS levels and the proliferation of Be-specific blood CD4⁺ T cells from CBD subjects. Be-induced T cell proliferation was inhibited by treatment with the thiol antioxidant N-acetylcysteine or the catalytic antioxidant MnTBAP. MnTBAP treatment also inhibited T cell proliferation in response to the unrelated, MHC class II-restricted antigen tetanus toxoid. Treatment of CBD blood lymphocytes, but not antigen presenting cells, with MnTBAP decreased Be-induced T cell proliferation > 40%. Conclusions: Be can mediate a thiol imbalance leading to oxidative stress that may modulate the proliferation and clonal expansion of Be-specific blood CD4⁺ T cells. These data suggest that Be-induced oxidative stress plays a role in the pathogenesis of granulomatous inflammation in CBD.