Central nervous system (CNS) prophylaxis involving cranial irradiation has frequently been found to result in cognitive deficits in survivors of childhood acute lymphoblastic leukaemia. The existence of this effect is still controversial, however, as is the question of factors that might affect its severity. The literature is surveyed and a meta-analysis undertaken on 30 IQ comparisons, which shows a substantial average decrement in irradiated subjects. The effect is larger when radiation is administered at a younger age and appears to be progressive. The effect may comprise two elements, one common to young cancer patients and one specific to CNS prophylaxis.