Cerebrospinal fluid was examined from 70 children with acute lymphoblastic leukemia for evidence of active myelin breakdown based on the release of myelin basic protein (MBP). Fifty-three asymptomatic children were followed from diagnosis with serial MBP determinations. Eight (15.1%) of 53 children had abnormal elevations of MBP, six of eight before receiving presymptomatic central nervous system therapy. Long-term observations are in progress. For comparison, six children with clinical and radiologic findings of leukoencephalopathy had abnormal MBP determinations, whereas no abnormalities were detected in 11 children with meningeal leukemia.