We examined argyrophil nuclear organizer regions (AgNOR) and proliferating cell nuclear antigen (PCNA) in 41 patients with surgically-treated thymic disease. AgNOR count and PCNA labeling index (LI) in thymic carcinoma were significantly higher than those in thymoma and thymic hyperplasia. A positive correlation was observed between the PCNA LI and argyrophilic nucleolar organizer region (AgNOR) counts within all thymic disease (r=0.31, P=0.002). The PCNA LI in invasive thymoma was lower than that in non-invasive thymoma. In survival analysis, the cut-off values for the PCNA LI and AgNOR count were chosen to produce two categories with equal numbers of 26 thymoma patients. There were no significant difference in the survival rate between the lower and higher group patients in relation with AgNOR count and PCNA LI. We conclude that combining AgNOR and PCNA may discriminate the biological activity of thymic disease. These staining methods can be performed with ease and, applied in a clinical laboratory on a routine basis to help predict cytological malignancy of thymic disease.