A great deal of attention has been focused on the antitumor effects of anti-MUC1 humoral and cellular responses. We examined whether anti-MUC1 antibody is present in patients with lung cancer, and evaluated its prognostic value. Serum was obtained from 30 patients with nonresectable, non-small cell lung cancer (NSCLC) and 60 healthy volunteers. The presence of anti-MUC1 antibody was determined by enzyme-linked immunosorbent assay. The patients were observed for a median follow-up time of 54.0 mo. Overall survival was estimated by the Kaplan-Meier method. Multivariate analyses were performed using the Cox proportional hazards regression model. Anti-KL-6/MUC1 antibody levels of the patients were significantly lower than those of normal individuals (p < 0.001). One-year survival rate of patients with high concentrations of anti-KL-6/MUC1 antibody was significantly higher than that of patients with low levels of anti-KL-6/MUC1 antibody (90.9% versus 21.1%, p < 0.001). Anti-KL-6/MUC1 antibody status was most strongly correlated with mortality, followed by lymph node status and albumin levels, whereas sex, serum lactate dehydrogenase (LDH), and carcinoembryonic antigen (CEA) levels, and metastasis status did not correlate with mortality. These preliminary results indicate that the degree of decrease in antibody level may be associated with a patient's prognosis.