There is growing concern about the oncogenic potential of growth hormone (GH) used therapeutically. In rat experiments, a variety of malignant tumours have been induced following administration of supraphysiological doses of GH, whilst in other studies in hypophysectomized animals a lower than normal incidence of carcinogen-induced neoplasms was reported. In acromegaly, in which there is a pathologically sustained high GH level, there is a significantly increased incidence of cancer in general and specifically of colonic neoplasia. To determine whether the use of GH in the treatment of radiation-induced GH deficiency causes tumour recurrence, a comparison was made of tumour recurrence rates between 47 children treated with GH for radiation-induced GH deficiency after treatment for a brain tumour and a control population from the North West Children’s Cancer Registry who did not receive GH (n = 160). All cases of acute lymphoblastic leukaemia (ALL), including those that were (n = 15) and were not (n = 146) treated with GH were reviewed. The computerized tomography (CT) scans in the children with brain tumours were reviewed at the time of GH commencement and subsequently. There were 5 brain tumour recurrences after GH therapy: 1 astrocytoma, 2 ependymomas and 2 medulloblastomas. Adjusting for variables other than GH which might affect tumour recurrence, the estimated relative risk of tumour recurrence was 0.82 (95% confidence interval: 0.28-2.37). In each tumour category there was no association between the use of GH and subsequent tumour recurrence. The only child treated with GH after treatment of ALL, who relapsed, had previously relapsed before GH therapy. Nineteen of 44 CT scans were abnormal at the time of GH commencement, 10 reported as demonstrating residual tumour. Two of the 5 children with brain tumours who subsequently relapsed had abnormal scans when GH was started and all 5 had abnormal scans after clinical relapse. Fourteen of the 39 children, who clinically remain relapse free, have had follow-up scans and in none has there been a deterioration in radiological appearance during GH therapy for a mean duration of 3.7 years. In our population there is no evidence of an increased risk of tumour recurrence following GH in replacement dosage, however, because of the confidence interval range, continued surveillance is essential. An abnormal CT scan is not a contraindication to GH therapy.