Background: Pubertal gynaecomastia is thought to be a clinical sign of an oestrogen-androgen imbalance, affecting up to 60% of boys. In most cases no underlying endocrinopathy can be identified. In boys, Anti-mullerian hormone (AMH) is produced by immature Sertoli cells and circulating level decreases as testosterone increases during pubertal maturation. In a previous cross sectional study we found significant lower levels of AMH in boys with pubertal gynaecomastia (Mieritz et al., Clin Endocrinol, 2013). Objective and hypotheses: To investigate serum AMH levels and genetic polymorphisms in boys with or without gynaecomastia. Method: 99 healthy Danish boys (aged 5.8-16.4 years) were followed in a prospective cohort over 8 years with semi-annual examinations (total examinations, n=951), including breast palpations and blood samples. Serum AMH concentrations were analysed by immunoassay (Beckman Coulter). Furthermore, we analysed two single nucleotide polymorphisms (SNPs) located in exon 1 of the gene encoding AMH (AMH rs10407022 (Table Presented)>G) and in a putative enhancer of the AMH-receptor (AMHR2 rs11170547 C>T) respectively. Results: Pubertal gynaecomastia was observed in 47/95 (49%) of the boys during follow-up. Circulating levels of AMH were significantly lower in boys with pubertal gynaecomastia compared to boys without - even after controlling for pubertal stage (P2) or AMH-receptor SNPs (CC vs CT, P=0.963). Conclusion: This is to our knowledge the first longitudinal study to find an association between low serum levels of AMH and the development of pubertal gynaecomastia. We speculate that this might be due to impaired testicular function in these boys.