The participation of T lymphocytes in the immunopathogenesis of corneal opacity in herpetic stromal keratitis was investigated. In BALB/c mice infected intracorneally with herpes simplex virus type 1 corneal opacity was manifested 10 days after infection, while in athymic mice corneas remained almost clear. Histologically, all opaque corneas revealed stromal edema accompanied by the diffuse presence of polymorphonuclear cells and lymphocytes. When complement (C’)-treated immune spleen cells were adoptively transferred into athymic mice 6 or 72 h after corneal infection, stromal keratitis with mild opacity was observed 10 days after transfer. The athymic mice given anti-Thy 1.2 + C’-treated immune spleen cells failed to develop corneal opacity. The difference, as revealed by light and electron microscopy, was the presence or absence of lymphocytic infiltration and edema in the posterior third layers of the stroma and endothelial lesions. The endothelium was infiltrated by lymphocytes or macrophages and showed various stages of destruction. The main cause of corneal opacity in the early stage of herpetic stromal keratitis is thought to be stromal edema due to an adverse effect on the endothelium by immune T lymphocytes.