Forty-one cerebrospinal fluid (CSF) specimens from children were investigated with a radioimmunoassay for their content of myelin basic protein (BP). Eight specimens were regarded as BP-positive (BP is greater than or equal to 1.0 ng/ml). Twenty-nine were BP-negative and 4 could not be analyzed because of an excessive protein content. The BP-positive samples were from 6 children with evidence of severe acute brain damage leading to death in 5 cases: i.e., 2 term newborns with perinatal asphyxia, a 4 week-old child with severe convulsions, a 3 year-old boy with hypoxia due to laryngitis, and a 12 year-old girl with encephalitis. One preterm baby survived severe hypoxia and developed hydrocephalus shortly afterwards. We conclude that BP becomes detectable in CSF of newborns and older children under certain pathological conditions, and that the presence of BP in CSF may be associated with severe brain tissue destruction.