OBJECTIVE The purpose of this study was to determine (1) the influence of estrogen deficiency induced by gonadotropin-releasing hormone (GnRH) agonist administration on insulin sensitivity as well as hormones and factors related to insulin resistance and (2) the differences in the influence for these parameters by the degree of basal insulin sensitivity. METHODS Thirty-five women diagnosed with leiomyoma were enrolled in this study. Serum levels of fasting glucose, insulin, sex steroid hormones, sex hormone-binding globulin (SHBG), vascular inflammatory markers and cytokines before and at 6months after commencement of GnRH agonist administration were examined. RESULTS In all women, levels of insulin, glucose and homeostasis model assessment of insulin resistance (HOMA-IR) were not significantly changed. However, in women who had a low HOMA-IR before treatment, levels of insulin, glucose and HOMA-IR showed significant increases and total testosterone level showed a significant decrease. In women who had a high HOMA-IR, levels of insulin, HOMA-IR and SHBG were significantly decreased and levels of highly sensitive C-reactive protein, soluble intercellular adhesion molecule-1, E-selectin and monocyte chemoattractant protein-1 were significantly increased. CONCLUSIONS Change in insulin sensitivity caused by GnRH agonist administration for premenopausal women with leiomyoma differs depending on baseline insulin sensitivity before treatment.