Lymphocyte subsets from 5 groups of patients with different types of chronic headaches were analyzed by flow cytometry to determine the number of cells bearing the following markers: T11 (E-rosette receptor), T3 (total T cells), T4 (T-helper/inducer cells), T8 (T-cytotoxic/suppressor cells), B1 (B-cells), NKH-1 (Natural killer cells), I2 (Class II histocompatibility antigens, monomorphic determinants) and Mo2 (monocytes). In 4 of 5 of these groups, the absolute number of T8 lymphocytes was significantly lower than that of control group. In addition, the T4:T8 ratio was higher in these 4 groups. These findings may suggest a relationship between the immune system and the cause of chronic headaches.