The growth inhibition of remote metastases by a primary tumor is known as endogenous growth inhibition leading to tumor dormancy. Such a phenotype has not been described in primary malignant gliomas. However, although glioma cells have frequently spread to other parts of the brain at the time of diagnosis, formation of solid secondary tumors is uncommon. We hypothesize that a dormant population of distant glioma cells exist. The purpose of this study was to investigate whether primary gliomas could inhibit secondary tumor formation. Subcutaneous tumors from human gliomas were grown as xenografts in Swiss nude mice. At a tumor size of at least one cm(3), the same amount of cells was injected into the contralateral flank or into the right cerebral hemisphere. Control mice without a primary tumor were injected with tumor cells either into the right flank, the right hemisphere, or bilaterally subcutaneously. Only one of 18 human gliomas demonstrated inhibition at the subcutaneous and intracerebral secondary implantation sites. Growth inhibition of the secondary tumors was accompanied by a significant reduction in microvessel density, upregulation of vascular endothelial growth factor mRNA and downregulation of basic fibroblast growth factor mRNA. Therefore, endogenous inhibition of secondary tumors may represent a rare phenotype in malignant glioma.