Cerebrospinal fluid (CSF) levels of immunoreactive parathyroid hormone (iPTH) and immunoreactive calcitonin (iCT) were measured by radioimmunoassay in 23 outpatient leukemic children on maintenance chemotherapy. These hormones were detectable in the CSF of all patients: iPTH 148 ± 11 pg/ml (mean ± SEM); iCT 14.3 ± 0.8 pg/ml. iPTH and iCT were also measured in serum (iPTH 396 ± 18 pg/ml; iCT 32.3 ± 1.4 pg/ml). CSF values were significantly lower (p < 0.001) than serum concentrations; no significant correlation between the two compartments was found. Our study indicates the presence of iPTH and iCT in the CSF of children.