The cancer-associated antigen NY-ESO-1 is expressed in a number of malignancies of different histological type. Patients with NY-ESO-1 expressing tumors have been shown to bear circulating autoantibodies against this antigen. In this study, we have assessed the NY-ESO-I autoantibody response in patients with lung cancer by a serum ELISA. Using a serum dilution of 1:400 we detected seroreactivity in 35 of 175 (20%) of patients. Incidence of autoantibodies was significantly higher in patients suffering from non small cell lung cancer (NSCLC, 23%) as compared to those with small cell lung cancer (SCLC, 9%). In the NSCLC group, NY-ESO-I antibody was significantly more frequent in patients with undifferentiated tumors (40%) as compared to patients with either adenocarcinoma or squamous cell carcinoma (15 and 29%). Our observations indicate that induction of NY-ESO-I autoantibodies depends on the histological subtype within a given tumor entity.