28 children with acute lymphoblastic leukaemia and 7 with malignant non-Hodgkin-lymphoma had a computerised tomography (CT) of the brain 2--84 months after the diagnosis was made. The treatment of these children included skull irradiation with a local dose of 1500--2400 R 60Co in the initial therapeutic phase and 4--5 intrathecal methotrexate injections in a dose of 10 mg/m2 body surface per dose during the phase of the skull irradiation. Abnormal CT results were found in 10 children. In some children CT investigations were also performed before the treatment to the CNS began. Abnormal findings before treatment remained unchanged in the follow-up investigations. Thus a large proportion of the abnormalities described had been present before treatment. It appears that pathological changes in the computer tomogram occur in a relatively large number of children after acute lymphoblastic leukaemia and non-Hodgkin-lymphoma, and that most of these changes can be found before CNS treatment. It is unlikely that prophylactic treatment to the CNS can cause pathological changes in the brain detectable by computerised tomography.