In forty-five children the hypoxanthine concentration in cerebrospinal fluid (CSF) was measured (fifty-two samples). In newborn infants (nineteen patients) the hypoxanthine levels were higher in patients with clinical conditions associated with hypoxia (idiopathic respiratory distress syndrome, asphyxia, apneic attacks) than in patients without clinical hypoxia (P less than 0.01). In hypoxic patients the hypoxanthine concentration varied between 5 and 28 mu mol/l. In children outside the neonatal period the hypoxanthine concentration in CSF varied considerably in different diseases. High levels were registered in meningitis prior to treatment, febrile convulsions and in lymphoblastic leukaemia, probably reflecting tissue hypoxia and an increased tissue catabolism.