Oral candidiasis is associated with defects in cell-mediated immunity and is common among patients undergoing cytotoxic chemotherapy, or corticosteroid or antibiotic therapy, and those patients seropositive for AIDS and HIV (human immunodeficiency virus). This paper demonstrates the important role of cell-mediated immunity in oral candidiasis in 2 cases of thymoma associated with myasthenia gravis. Both suffered from recurrent oral candidiasis after a thymectomy, radiotherapy, and chemotherapy. There was an initial good response to conventional antifungal therapy, which later became refractory. Lymphocyte subset quantitation showed a T cell deficiency and a decreased CD4/CD8 ratio. Levamisole, an immunomodulator, or an immunopotentiating drug was added as adjunctive therapy in combination with oral nystatin treatment. Oral candidiasis responded favorably, and substantial relief was obtained with a concurrent increase in T cells and the CD4/CD8 ratio. These findings clearly demonstrate a significant role of cell-mediated immunity in oral candidiasis, and that eradication of infection is dependent on the host defense mechanism.