Humoral immune responses against the “Cancer‐Testis” (CT) antigen NY‐ESO‐1 are frequently observed in patients with NY‐ESO‐1 expressing tumors. This is in contrast to other known tumor antigens (TA) defined by antibody or cytotoxic T cell (CTL) reactivity, i.e., MAGE‐1, MAGE‐3, SSX2, Melan A, and tyrosinase. No NY‐ESO‐1 antibody has been detected in healthy controls and patients with NY‐ESO‐1 negative tumors. In this study, we have assessed the NY‐ESO‐1 serum antibody response in patients with NY‐ESO‐1 positive tumors of different histological types and stages using Western blotting and an ELISA. Of the 12 patients analyzed, 10 had demonstrable NY‐ESO‐1 antibodies at the start of the study. All patients were followed for changes in NY‐ESO‐1 antibody titers during the course of tumor treatment and clinical evolution. In 4 patients, an increase of NY‐ESO‐1 antibody titer was observed with progression of disease or extensive tumor necrosis under treatment. One patient showed a stable NY‐ESO‐1 antibody titer over 3 years along with gradual regression of a large tumor mass. In 5 patients, a decrease of NY‐ESO‐1 antibody was detected: in 1 patient after curative tumor resection, in 3 patients with partial regression of metastatic disease under chemo‐ and immunotherapy, and in another patient with a NY‐ESO‐1 negative tumor relapse. Our results indicate that the induction and maintenance of NY‐ESO‐1 antibody is dependent on the presence of NY‐ESO‐1 expressing tumors. Furthermore, changes in NY‐ESO‐1 antibody titers correlate with the evolution of NY‐ESO‐1 positive disease. Int. J. Cancer (Pred. Oncol.) 84:506–510, 1999. © 1999 Wiley‐Liss, Inc.