The MAGE-4 gene, a member of the MAGE gene family, is expressed in various cancers, including head-and-neck squamous-cell carcinomas (HN-SCC), but is not expressed in any normal tissues except for the testis and placenta. The aim of this study was to determine whether serum MAGE-4 protein is a useful tumor marker for detection of HN-SCC. An enzyme-linked immunosorbent assay was used to measure serum level of MAGE-4 protein. The serum level of MAGE-4 in pre-operative HN-SCC patients was significantly higher than that in patients with non-malignant diseases (NMD) of the head and neck, volunteers undergoing cancer screening (VOL), or healthy donors (HD). When the cut-off level was determined at 1.15 ng/ml (mean plus 3 SD of HD), sera from 28 of 96 patients with HN-SCC (p < 0.0001 vs. the other groups), 7 of 82 patients with NMD, 2 of 92 with VOL, and 0 of 68 HD were positive for MAGE-4. Serum levels of MAGE-4 protein in all 7 HN-SCC patients whose sera were positive for MAGE-4 before operation decreased after operation, and, in one patient, a renewed rise in serum level was followed by recurrence. These results indicate that MAGE-4 protein is detectable in sera of a significant number of HN-SCC patients, and that serum MAGE-4 protein might be a useful tumor marker to monitor the recurrence of MAGE-4-positive HN-SCC.