Bladder cancer (BC) is a common tumor of the urinary system. According to the latest global cancer data in 2021, the incidence of BC ranks thirteenth among all tumors worldwide, and in the United States, the incidence of BC in men is the fourth highest, after prostate cancer, lung cancer, and colorectal cancer [1]. The occurrence of BC is influenced by a variety of internal and external factors related to genetic susceptibility, environment and so on, but the etiology and pathogenesis are still not completely clear.

Autoimmune diseases (AIDs) are a group of chronic diseases caused by the immune system's loss of immune tolerance to self-antigens, which further induces tissue damage and inflammatory responses and ultimately leads to target organ damage [2,3]. These include systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), type 1 diabetes (T1D), multiple sclerosis (MS), inflammatory bowel disease (IBD), myasthenia gravis (MG) and other chronic diseases, and IBD is classified into Crohn's disease (CD) and ulcerative colitis (UC) according to its pathogenesis.

Mendelian randomization (MR) is the use of single nucleotide polymorphism (SNP) as a genetic instrumental variable (GIV) to estimate the causal effect of exposure and outcome [4]. MR is relatively independent of factors such as social environment and individual lifestyle, and has chronological rationality in causal inference, which can overcome the limitations of traditional epidemiological methods for observational studies (limited sample size, heterogeneity of demographic characteristics, potential confounders, the interference of reverse causation and other factors), and lead to more plausible findings [5,6]. Therefore, this study investigated the causal association between AIDs and BC by MR methods, with a view to demonstrating whether which AIDs increase the risk of BC.