The neural cell adhesion molecule (NCAM) was recently suggested as a marker for small-cell lung cancer (SCLC). Immunohistochemical analysis demonstrated the presence of the NCAM in 78% of SCLC patients and in 25% of patients with other cancer forms. NCAM was proposed to be the most sensitive marker for SCLC, and it may also be an important prognostic marker for SCLC. We used a competitive ELISA to analyze the concentrations of NCAM in sera from 96 SCLC patients, 16 patients with non-SCLC, 4 patients with other cancer forms, and 16 healthy controls. All sera were collected at the time of diagnosis, before the patients received chemotherapy. The polyclonal antibody used in the assay recognized all three isoforms of NCAM. The concentration of NCAM was related to clinical parameters of the patients such as age, sex, blood group status, stage of disease, organ site involvement of metastases, survival, and expression of the ganglioside fucosyl-GM1 (FucGM1). Sera were considered positive if NCAM concentrations were higher than the mean concentration of healthy controls plus two standard deviations. Twenty-two percent of the sera from SCLC patients were positive for NCAM. No difference in concentration was found between SCLC patients with localized and extensive disease. Serum from one patient with cancer of the thyroid, but no sera from non-SCLC patients or normal healthy controls, was positive. The expression of NCAM did not correlate to any of the clinical parameters, and no correlation was found to the other serum marker, FucGM1.(ABSTRACT TRUNCATED AT 250 WORDS)