STUDY QUESTIONAre anti-Müllerian hormone (AMH) levels reduced in girls with newly diagnosed cancer before the start of treatment?SUMMARY ANSWERAMH levels are already compromised in girls at the time of cancer diagnosis compared with healthy girls.WHAT IS KNOWN ALREADYIn women diagnosed with cancer, evidence of reduced ovarian function has been described even before treatment has started. In girls with newly diagnosed cancer, no data are available.STUDY DESIGN, SIZE, DURATIONWe performed an age-matched case-control study in girls with newly diagnosed cancer.PARTICIPANTS/MATERIALS, SETTING, METHODSWe determined serum AMH levels in a cohort of 208 girls with newly diagnosed cancer, up to 18 years of age at diagnosis, and compared them with AMH levels of 250 age-matched healthy girls. The diagnoses included were acute lymphoblastic leukaemia, acute myeloid leukaemia, Hodgkin lymphoma, non-Hodgkin lymphoma, nephroblastoma, sarcoma and neuroblastoma.MAIN RESULTS AND THE ROLE OF CHANCEThe median age was 6.6 years (range 0.0-17.4), comparable with that in the control group (median 6.3 years, range 0.3-18.0). Girls with childhood cancer presented with significantly lower serum AMH levels compared with healthy age-matched controls (standard deviation scores (SDS) -0.8, P < 0.001). Median AMH level in patients was 1.4 μg/l (0.1-10.2) versus 3.0 μg/l (0.1-18.3) in controls. Specifically, 84% of all patients had AMH levels below the 50th percentile of normal AMH levels, and 19% below the 10th percentile. Surrogate markers of general health status (temperature, C-reactive protein and haemoglobin levels at diagnosis) were significantly correlated with AMH SDS.LIMITATIONS, REASONS FOR CAUTIONSome caution is warranted because AMH levels increase with age in healthy children but the cases and controls were age-matched in our study. Although our sample size was large, additional studies are still required in an independent cohort.WIDER IMPLICATIONS OF THE FINDINGSOur study shows that AMH levels are reduced in girls with newly diagnosed cancer even before the cancer treatment has started. AMH levels correlate with impairment of general health status in girls. Therefore, besides (pre) antral follicle number, other factors may influence serum AMH levels. Longitudinal studies during and after childhood cancer are currently being performed in order to evaluate possible ovarian recovery after discontinuation of treatment.STUDY FUNDING/COMPETING INTEREST(S)W.v.D. is supported by the Paediatric Oncology Centre Society for Research (KOCR), Rotterdam, The Netherlands. J.S.E.L. has received grants from the following companies (in alphabetical order): Ferring, Genovum, Merck Serono, Merck Sharp and Dome, Organon, Serono, Shering Plough and Shering. All other authors have nothing to disclose.