PURPOSE: The aim of this study was to investigate the causal relationship between low-density lipoprotein cholesterol (LDL-C) and five cancers (breast, cervical, thyroid, prostate and colorectal) using the Mendelian Randomization (MR) method, with a view to revealing the potential role of LDL-C in the development of these cancers. METHODS: We used gene variant data and disease data from the Genome-Wide Association Study (GWAS) database to assess the causal relationship between LDL-C and each cancer by Mendelian randomisation analysis methods such as inverse variance weighting and MR-Egger. Specifically, we selected Proprotein convertase subtilisin/kexin type 9 (PCSK9) and 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), genes associated with LDL-C levels, as instrumental variables, extracted the corresponding single nucleotide polymorphism (SNP) data and analysed the associations of these SNPs with five cancers.In addition, sensitivity analyses and heterogeneity tests were performed to ensure the reliability of the results. RESULTS: The analyses showed that when using HMGCR gene, LDL-C were significantly and positively associated with breast (OR:1.200, 95% CI:1.082-1.329, p = 0.001), prostate (OR:1.198, 95% CI:1.050-1.366, p = 0.007), and thyroid cancers (OR:8.291, 95% CI:3.189- 21.555, p = 0.00001) were significantly positively correlated, whereas they were significantly negatively correlated with colorectal cancer (OR:0.641, 95% CI:0.442-0.928, p = 0.019); the results for cervical cancer were not significant (p = 0.050). When using the PCSK9 gene, LDL-C levels were significantly and positively associated with breast (OR:1.107, 95%:CI 1.031-1.187, p = 0.005) and prostate (OR:1.219, 95%:CI 1.101-1.349, p = 0.0001) cancers, but not with cervical (p = 0.294), thyroid cancer (p = 0.759) and colorectal cancer ( p = 0.572). CONCLUSION: Analyses using both the HMGCR and PCSK9 genes have shown that LDL-C may be a potential risk factor for breast and prostate cancer, while analyses of the HMGCR gene have also suggested that LDL-C may increase the risk of thyroid cancer and decrease the risk of colorectal cancer.