Tumour relapse rates in 14 patients with medulloblastoma, 8 with glioma, 2 with ependymoma, 6 with leukaemia, and 1 with T-cell lymphoma who received growth hormone (GH) treatment for growth failure secondary to cranial irradiation were compared with rates among patients treated with radical radiotherapy for the same types of tumour. Five relapses (in 5 patients) occurred (1 optic nerve glioma, 2 medulloblastomas, and 2 ependymomas), three during and two after completion of GH treatment. Patients with medulloblastoma and ependymoma who relapsed were older at tumour diagnosis, underweight at the start of GH therapy, and entered puberty later than similar relapse-free patients. The late relapse rate of medulloblastoma and glioma was unaltered by GH therapy. Ependymoma carries a poor prognosis, and of the 4 late survivors, the 2 who received GH relapsed. No leukaemic relapse has been associated with GH treatment. The findings indicate that GH therapy does not increase the relapse rate of medulloblastoma, glioma, and leukaemia.