Phenylbutyrate is an aromatic fatty acid that is converted to phenylacetate in vivo by mitochondrial Beta-oxidation to phenylacetate. Preclinical studies have shown that continuous exposure to phenylacetate or phenylbutyrate can induce tumor cytostasis and differentiation in a wide variety of cell lines including malignant gliomas and neuroblastomas. However, phenylbutyrate has been shown to be a more potent differentiating agent than phenylacetate in a variety of tumor cell lines. In addition, phenylbutyrate appears to have molecular activities that are distinct from phenylacetate and may induce apoptosis. Phase I trials in adults and children have established the feasibility of administering phenylbutyrate on a prolonged schedule. A phase II trial of phenylbutyrate administered as a continuous intravenous infusion will be performed in children with recurrent or progressive brain tumors.