Rhabdomyosarcoma (RMS) cells express the polysialylated (PSA) form of the neural cell adhesion molecule (NCAM). During embryogenesis, PSA-NCAM is widespread and dynamically regulates embryonal developing processes, whereas postnatally, PSA-NCAM becomes restricted to a few regions of neural plasticity and regenerating neural tissues. Recently, PSA-NCAM has been shown to be a diagnostic and prognostic marker in adult patients with small cell lung cancer and multiple myeloma, both PSA-NCAM-expressing tumors. In this study, we determined the amount of PSA-NCAM in tumor specimens of nine children with different histologic types and clinical stages of RMS immunohistochemically, using the polysialic acid-specific MAb 735. In seven children, serum levels were investigated by an immunoluminescence assay using the same MAb. Patients with extensive disease showed strong staining of the tumor specimens, whereas patients with limited stages or after chemotherapy had distinctly a lesser amount of PSA-NCAM or almost no staining. Simultaneously, the serum levels were very high (up to 9-fold) in patients with extensive disease, whereas patients with limited disease or after successful therapy had normal serum levels. We conclude that PSA-NCAM expression is high in tumor specimens and serum of patients with advanced stages of RMS and decreases during successful therapy. PSA-NCAM might therefore serve as a marker for diagnosis and monitoring childhood RMS.