Abstract The unexplained regression of a metastasis of renal cell carcinoma (RCC) is rare phenomenon. While an immune mechanism has been proposed for spontaneous or unexplained regression of RCC, only limited data are available regarding immunological response. We report a case of a 62-year-old man with RCC whose paravertebral pleural tumor regressed after the withdrawal of interferon-α. In the present case, we demonstrate the enhancement of T cell proliferative response against autologous RCC cells secondary to the unexplained regression using the mixed-lymphocyte tumor culture. To our knowledge, the enhancement of an antitumor immunity secondary to an unexplained regression of RCC has not previously been reported.