Nineteen children with mumps meningitis were investigated. Peripheral blood T cell subset function and number were tested during the acute phase of meningitis and again when the children recovered. No significant difference in the total number of T cells, or T helper: suppressor lymphocyte cell ratio was found in the children with mumps meningitis as compared with the controls. However, a significant increase in the number of T suppressor/cytotoxic cells in the peripheral blood of children with mumps meningitis in the acute phase was found. This increase in the T suppressor subset was still present even after the children recovered from the disease. Two children who showed deafness had the highest increases in the T suppressor/cytotoxic lymphocyte subset. Functional analysis of T suppressor cell activity demonstrated a significant suppressive activity of lymphocytes obtained during the acute phase of the disease. However, after recovery the suppressive activity found was not significant (P greater than or equal to 0.1).