Intracerebral inoculation of lymphocytic choriomeningitis virus (LCMV) generally results in a fatal T cell-mediated meningitis. In a previous study we have demonstrated a compromised blood-brain barrier (BBB) under such conditions. Using semi-quantitative radiography and the low molecular tracer 2-amino-[1-14C]isobutyric acid we now demonstrate an uncompromised BBB in i.c. infected T cell-deficient nu/nu mice, but serious dysfunction in heterozygous littermates. Transfer experiments were used to characterize and compare the cell subset(s) involved in inducing BBB dysfunction and fatal disease. It was demonstrated that Thy-1+, CD8+ class I-restricted T cells were mandatory for the increase in BBB permeability as well as for mortality. In addition, depletion of class II-restricted CD4+ cells significantly weakened both effects of cell transfer. These results support the idea of a causal relationship between virus-specific T cell activity, BBB dysfunction, and fatal disease. Furthermore, the data indicate that T helper cells augment the response of LCMV-specific CD8+ effector cells.