Leukotriene D4 (LTD4) is an inflammatory mediator synthesized from LTC4 via gamma-glutamyltransferase (GGT) enzyme in the arachidonic acid pathway and has been reported to induce cell proliferation and survival in cancer. In recent studies, it has been shown that there is an increase in GGT enzyme activity in breast cancer. In recent studies, it has been shown that there is an increase in GGT enzyme activity in breast cancer. The aim of this study is to determine whether there is a change in serum LTD4 levels in patients with breast cancer and to examine the relationship between LTD4 and GGT. For this purpose, serum samples were taken from 43 patients diagnosed with breast cancer and eight healthy controls. Patients were divided into five subgroups, Luminal A, Luminal B, Luminal B-HER2(+), HER2(+), and triple-negative. LTD4 levels were measured by the ELISA method. Mean levels of LTD4 in the patients were significantly higher than in healthy controls (p < 0.05). Based on the molecular subtypes, serum LTD4 levels were found to be considerably higher in the Luminal A, Luminal B, and Triple (-) subgroups than in the controls (p <0.01, p <0.005, and p <0.005, respectively). Higher levels of LTD4 have been observed in post-menopausal patients than in premenopausal patients (p <0,05). A statistically significant positive correlation was observed between GGT activity and LTD4 levels in the whole study group and post-menopausal patients (R=0.349, p=0.014, and R=0.437, p=0.042, respectively). According to the literature, this study is the first to examine LTD4 levels in breast cancer and supports other studies showing the role of leukotrienes in cancer. Because of LTD4’s ability to induce proliferation and inhibit apoptosis, increased levels of LTD4 in our study may be associated with cancer development, especially in post-menopausal women.