Objective: To explore the causal relationship between asthma, allergic rhinitis (AR), and chronic sinusitis (CRS), using two sample Mendelian randomization (MR) analysis, thereby providing foundational evidences for the pathogenesis and treatment of CRS. Methods: The genetic variations in AR and asthma were used as instrumental variables, with genetic data from the Integrated Epidemiology Unit (IEU) Open database. A total of 14 283 asthma and 18 934 AR cases were included, with 98 300 and 64 595 corresponding normal control cases, respectively. For CRS, there were 3 236 CRSwNP and 8 524 CRSsNP, respectively, with 167 849 and 167 849 corresponding normal control cases, respectively. The genetic data were analyzed using the inverse variance weighting method (IVW), MR Egger method, weighted median method, and Cochran's Q-test. Results: The IVW analysis showed that asthma increased the risk of both CRSwNP (OR=482.8, 95%CI: 57.18-4 077.78, P<0.001) and CRSsNP (OR=25.73, 95%CI: 9.79-67.56, P<0.001); AR significantly increased the risk of CRSsNP (OR=5.40, 95%CI: 1.68-17.26, P=0.004), but not CRSwNP (OR=7.38, 95%CI: 0.80-67.73, P=0.077). Conversely, neither CRSwNP nor CRSsNP increased the risk of asthma or AR. Conclusion: According to Mendelian genetic laws, asthma is a risk factor for CRSwNP and CRSsNP, while AR is a risk factor for CRSsNP.