Peripheral T-lymphocyte subsets: T-helper (OKT4) and T-suppressor (OKT8) cells were studied quantitatively in 20 patients with recurrent aphthous ulceration (RAU) in ulcerative, as well as inactive, stages of the disease. The figures were compared with T-lymphocyte subsets from matched control donors with no history of RAU. The ratio of T-helper: T-suppressor cells was significantly lower in both stages in the patients compared with controls due to a significantly increased number of T-suppressor cells in RAU patients. The number of T-helper cells in the patients did not differ significantly in either stage compared with controls. The study support the hypothesis of recurrent aphthous ulceration being a disorder of immunodeficiency.