Ovarian function was assessed in 40 long term survivors who had received standard United Kingdom Acute Lymphoblastic leukaemia (UKALL) protocols and were in first clinical and haematological remission. A menstrual and pregnancy history was taken (median age at assessment: 18.8 (12-34.7) years) and the acquisition of adult secondary sexual characteristics confirmed in each patient. Basal bloods were taken for follicle stimulating hormone (FSH), luteinizing hormone (LH), and serum oestradiol estimations. Serum progesterone concentration was measured in those patients who were in the luteal phase of their menstrual cycle at assessment. In addition, menstrual cycle profiles of salivary progesterone concentrations were derived from daily samples in 12 patients. All patients achieved adult sexual development; median age at menarche was early at 12.4 (9.0-14.6) years and 37 of them have regular menses. Ten patients have had 14 live births, and evidence of ovulation was seen in a further 11 patients assessed in the luteal phase of the menstrual cycle. Four patients had damaged ovaries, two of whom show evidence of ovulation; three of the four received craniospinal irradiation and one received cyclophosphamide as part of her chemotherapy regimen. None of these patients has yet developed total ovarian failure or required sex steroid replacement therapy. The medium term outlook for ovarian function is good for the majority of childhood ALL survivors. The spinal component of craniospinal irradiation is a major risk factor for ovarian damage, and cyclophosphamide may be a contributory factor. A premature menopause remains a possibility if significant follicular depletion has occurred at the time of cytotoxic treatment.