Cranial radiation therapy in childhood acute lymphoblastic leukaemia has been associated with adverse neuropsychological effects, such as low intelligence. However, records show that these associations usually occur when the dose of radiation used is 2400 cGy. We investigated whether a lower dose of 1800 cGy had the same adverse effects on long-term survivors and whether high doses of methotrexate but no radiation therapy would have a more beneficial effect. We evaluated 203 children for six years in a multi-centre European study. The patients were divided into two groups: 129 children treated with 1800 cGy of cranial radiation therapy and 74 children who received high-dose methotrexate but no radiation therapy. We used full scale intelligence quotient, verbal, and performance IQ tests to assess the patient's intelligence. We found a significant decline in full scale intelligence quotient in the irradiated group that increased with the length of time from diagnosis. Younger age at diagnosis was associated with lower full scale intelligence quotient in the radiated group. Our results indicate that a radiation dose of 1800 cGy can have negative effects on neurocognitive function and we continue to question the benefit of low-dose cranial radiation therapy.