The transmembrane 4 superfamily member KAI1/CD82, a metastasis suppressor, is correlated inversely with the progression and invasion of several tumors. It is capable of inhibiting metastasis without affecting tumorigenicity per se. KAI1/CD82 expression is down-regulated in the progression of common solid epithelial tumors of adulthood. Mutation of p53 is suggested to be involved in the modulation of KAI1. As little is known about its expression and possible prognostic impact in pediatric tumors, we investigated KAI1/CD82 expression in cell lines and primary tumor samples from pediatric tumors of neuroectodermal origin, neuroblastoma and Ewing's sarcoma family tumor. Twenty-four of 29 Ewing's sarcoma family tumor cell lines, independent of p53 status, showed KAI1 mRNA positivity by reverse transcription-PCR analysis in contrast to zero of eight neuroblastoma cell lines. Among 13 primary Ewing's sarcoma family tumor samples from patients with different disease extension, KAI1 mRNA expression was low as detected by reverse transcription-PCR. Twenty of 30 primary neuroblastoma specimens were KAI1-negative by immunofluorescence analysis whereas the remaining 10 gave weak to moderate staining patterns. There was no apparent correlation of KAI1 expression with any clinical or genetic features of the patients whose tumor samples were studied. Consequently, KAI1 may not be of prognostic relevance in this group of tumors although there may be some role for KAI1 modulation in the biology of these neuroectodermal tumors.