Breast cancer (BC) has become the main malignant tumor threatening the health of women worldwide. Previous studies have reported that Lactate dehydrogenase-A (LDHA) has critical roles in cancer development and progression. We aimed to explore the roles of LDHA and LDH5 isoenzyme activity in BC, which provides a new insight into LDHA for the treatment of BC.The expression of LDHA in BC and its relationship with clinicopathological features were obtained from various databases including The Cancer Genome Atlas (TCGA), Human Protein Atlas (HPA), Breast Cancer-Gene Expression Miner (bc-GenExMiner), TNMplot, UALCAN. The Kaplan‒Meier Plotter was used to evaluate the prognostic value of LDHA. Western blot was performed to detect LDHA expression. Agarose gel electrophoresis was performed to detect the activities of LDH isoenzymes. The in vitro proliferation, migration and invasion potentials of BC cells were evaluated using MTT assays, colony formation, wound-healing assay, matrix metalloproteinase assays and transwell assays, respectively. The activities of LDH isoenzymes in serum and tissues were measured in patients with BC and healthy controls.Compared to normal tissues, LDHA expression was significantly higher in BC tissues. Patients' nodal status, histological types, TP53 mutation status and PAM50 subtypes were significant factors influencing the LDHA expression. By overexpressing or silencing LDHA gene in BT549 cells, it was confirmed that LDHA promoted cell proliferation, migration and invasion. LDH5 isoenzyme activity in patients with BC was higher than healthy controls. The increased activity of LDH5 isoenzymes was induced by overexpression of LDHA in BC. High expression of LDHA was found to be associated with poor prognosis in BC.LDHA plays a critical role in the progression of BC through the regulation of the activity of LDH5 isoenzyme, indicating that LDHA may serve as a valuable target for BC treatment.© 2025 Fang et al.