A 46-year-old female was admitted to our hospital on December 1991 for further examination of an abnormal shadow on chest X-ray. A huge mass in the upper mediastinum and a thin-walled cavitary lesion in the right upper lobe were found by chest radiography and computed tomography. On March 2, 1992, surgical resection was performed with pathological diagnosis of thymoma for the mediastinal mass and pulmonary cryptococcosis for the cavitary lesion. Although ATL cells were not found in the peripheral blood, anti-HTLV-1 antibody was found to be positive. Natural killer cell activity was lower than normal control and other tests of humoral and cell-mediated immunity were normal. Histological examination of the specimens obtained from the thymoma revealed lymphocytic predominance without atypical cells such as ATL cells, but with integration of HTLV-1 proviral DNA. There seemed to be a causal relationship between HTLV-1 carrier state and thymoma. Since there are no reported cases of HTLV-1 carrier associated with pulmonary cryptococcosis and thymoma, we report the first case with a review of the literature.