Background  Childhood Brain Tumor Survivors (CBTS) are at increased risk of hypothalamic-pituitary (HP) dysfunction, caused by the tumor itself, radiotherapy , or brain injury. Being overweighted or underweighted at brain tumor diagnosis may reflect early hypothalamic dysfunction and be a risk factor to develop HP dysfunction during follow-up. The objective of this study was to examine whether BMI at brain tumor diagnosis is associated with the development of HP dysfunction, independent of brain tumor treatment. Methods: .Children surviving a brain tumor, excluding craniopharyngioma or a pituitary tumor, for minimally two years, in whom BMI at diagnosis of their brain tumor had been recorded were included from a previous cohort study (n=685 of 718). Odds-ratios (OR) for developing HP dysfunction were calculated using multivariable logistic regression, including BMI at diagnosis, gender, age at follow-up, radiotherapy, histology, location of the primary tumor, hydrocephalus and state of disease.Results.Of all, 10.8% (n=74) was overweighted ( BMI > + 2 SDS) and 5.0% (n=34) was underweighted ( BMI < -2 SDS) at diagnosis. Median follow-up time was 7.1 years (5.0-9.6). Being overweighted or underweighted at diagnosis was significantly associated with the development of HP dysfunction (OR 2.43, 95% CI 1.46-4.05) and obesity at follow-up (OR 3.75, 95% CI 2.36-5.97).Development of HP dysfunction was also significantly associated with radiotherapy, and location of primary tumor.In children not treated with radiotherapy (n=430), underweight and overweight were also both significantly associated with HP dysfunction (OR 2.91, 95% CI 1.16-7.28) and precocious puberty (3.00 , 95% CI 1.12-8.01). In this group, other significant risk factors for HP dysfunction included tumor location, hydrocephalus and state of disease at follow-up.Conclusions Being overweighted or underweighted at brain tumor diagnosis in childhood seems to be associated with the development of anterior pituitary deficiencies, posterior pituitary deficiencies and CPP, independently of given treatment.