Eighteen patients with metastatic renal cell carcinoma, who were treated by vaccines for active specific immunotherapy, also completed skin testing with autologous tumor cells, both prior to and following vaccine treatment. All patients have now been followed for more than 5 years. Ten patients who remained skin-test-negative following treatment had no clinical responses, and all had expired by 22 months. Eight patients became skin-test-positive; three of these had clinical regressions and three remain alive after more than 69 months. The survival times of the skin-test-positive group were significantly superior to those of the skin-test-negative group. The results suggest that skin testing with autologous tumor cells may accurately identify those patients who have acquired antigen-specific cell-mediated antitumor immunity.