Leukocyte functions were investigated in patients with oral candidiasis (OC), recurrent aphthous ulcers (RAU) and oral lichen planus (OLP). Blastogenesis and lymphokine-activated killer (LAK) activity were decreased in RAU and OLP compared with healthy controls, and lymphocytes from patients with OC exhibited the lowest blastogenesis and LAK activity. Neutrophil superoxide (O2-) generation showed the following relationship: healthy controls > RAU congruent to OLP > OC patients. No great differences in phagocytosis or natural killer (NK) activity were observed among patients and healthy controls, although NK activity in controls was slightly higher than in patients. In OC, all suppressed leukocyte functions improved with the disappearance of the clinical manifestations. However, in RAU there were no significant differences before and after treatment except for O2- generation. These results indicate that OC tends to occur in individuals predisposed to fungal infections and that the pathophysiology of both RAU and OLP is associated with leukocyte suppression.