Adult T-cell leukemia/lymphoma (ATL) is malignancy of mature T cells that caused by infection with human T-cell leukemia virus type I (HTLV-I). Leukemogenesis of ATL cells considered to involve a multistep oncogenic process, resulting in a very long latency period. But, we report here the case of a 21-year-old man having suffered from recurrent stomatititis who has already developed acute-type ATL. ATL generally occurs after a long latency period, and the present case in a young man is thus very rare.