We report a boy with acute lymphoblastic leukaemia (ALL) treated with chemotherapy and prophylactic cranial irradiation to a dose of 24 Gy. Six years after diagnosis he developed a glioma and died. Prior to 1979, four cases of second malignant neoplasm (SMN) of the brain had been reported in children treated for ALL. These SMNs occurred within 2 years of the original diagnosis (median 1.3 years) and at least two of four patients had not received prior radiotherapy. Since 1979, 28 cases of SMN of the brain have been reported including nine of 468 (1.9%) long-term survivors in one study. All occurred more than 3.7 years from diagnosis (median 6.5 years; range 4-13 years) and all received cranial irradiation (median 24 Gy; range 20-48 Gy). These data indicate a change in the pattern of SMNs which is most likely due to the introduction of cranial irradiation. As well, the frequency of SMNs in children treated for ALL appears to have increased, although it is still no greater than the risk of SMNs developing following the treatment of any other primary childhood neoplasm.