Background: Idiopathic hypogonadotropic hypogonadism (IHH) is a form of male infertility caused by a congenital defect in the secretion or action of gonadotropin-releasing hormone from the hypothalamus. Oestradiol emerged as the main sex steroid in the regulation of the hypothalamic–pituitary–testicular axis, reproductive function and growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis in men. Moreover, GH/IGF-1 axis has been suggested to play a role in IHH. Aims: This study evaluated serum IGF-1 in IHH men and controls. Furthermore, we evaluated the association between serum total oestradiol (TE2) and IGF-1 levels in patients and controls. Parameters including age, body mass index and fertility history were analysed. Settings and Design: This prospective study was conducted at the Royan institute. Materials and Methods: In 20 men with IHH and 20 controls, serum IGF-1 levels were estimated using chemiluminescence immunoassay and serum E2 levels were assessed by means of the electrochemiluminescence method. Statistical Analysis Used: Kolmogorov-Smirnov test, parametric t-test or the Mann-Whitney and the Pearson correlation coefficient were performed. SPSS version 22 was used for the analysis of data. Results: There was a significant decrease in serum IGF-1 levels in IHH patients compared with controls (145.1 ± 8.9 ng/ml vs. 229.6 ± 7.3 ng/ml P < 0.001, respectively). Furthermore, a significant decrease was observed in TE2 levels in IHH male patients (12.3 ± 2.5 pg/ml) compared with controls (31.9 ± 5.3 pg/ml P < 0.001). A positive correlation was observed between serum IGF-1 and TE2 levels in the total number of participants, suggesting that E2 deficiency in IHH cases can explain the lower levels of serum IGF-1. Conclusions: These findings suggest that the reduction in IGF-1 levels may be associated with the influence of E2 on the GH/IGF-1 axis, and may confirm the role of the GH/IGF-1 axis in IHH. Further investigations will be required to determine the exact mechanisms by which E2 and IGF-1 affect the reproductive neuroendocrine function.