Much evidence suggests that recurrent oral aphthous ulceration (RAU) is an immunologically mediated disease. Tumour necrosis factor has multiple biologic properties, some of which may be relevant to the pathogenesis of RAU. This study has assessed its production by peripheral blood leukocytes from aphthous patients in active and remission phases of disease and from patients with non‐aphthous ulceration (diseased controls). Each ulcer patient was studied in parallel with a matched healthy control volunteer. A bioassay against the standard mouse fibrosarcoma line, L929, was used to assess the levels of tumour necrosis factor‐a (TNF). Significantly greater amounts of TNF were released from unstimulated monocyle‐enriched and monocyte‐depleted leukocyte fractions in active RAU compared with those from healthy control donors, suggesting that this cytokine may be associated w