The clinical value of cerebrospinal fluid (CSF) polyamine determinations in childhood medulloblastoma has been suggested. We performed 72 CSF polyamine determinations in 35 children with primary brain tumors. Spermine values were normal and spermidine values were inconsistently elevated. CSF putrescine values, however, were consistently elevated in patients with histologically malignant brain tumors: medulloblastoma, ependymoma, pineal germ cell tumors, primitive neuroectodermal tumors, and brainstem gliomas. Children with supratentorial astrocytomas had normal CSF polyamine values. CSF putrescine values were closely correlated with clinical state, with the highest concentrations identified in patients with widely disseminated recurrent disease. We found CSF putrescine to be a sensitive indicator of active disease in childhood malignant brain tumors. Further investigation is warranted into the predictive value of CSF polyamines in determining tumor relapse before clinical or other diagnostic studies reveal recurrent disease.