Measurements have been made of the growth hormone (GH) responses of nine children with acute lymphoblastic leukaemia to the insulin and arginine tolerance tests. All the patients had received induction treatment with prednisone, vincristine and doxorubicin ('Adriamycin') for 4 weeks followed by 2400 rad of orthovoltage cranial irradiation plus five intrathecal injections of methotrexate. During the study all the children were in complete remission, which had been maintained with 6-mercaptopurine and methotrexate for 4 to 26 months. No pulses of steroids were used in remission. Five patients in early remission did not respond to either stimulation test, three having longer remissions showed a late and low response to the insulin test and the one patient with the longest remission (26 months) had a normal GH response. Heights and bone ages were normal. These results, which suggest the possibility of a gradual recovery after the end of CNS treatment including orthovoltage cranial irradiation, are contrasted with those for megavoltage treatment reported by Shalet et al. (Shalet, S.M., Beardwell, C.G., Morris-Jones, P.H., Pearson, D., Archs. Dis. Childh., 1976, vol. 51, 489).