Background: Interleukin-37 (IL-37) is a recently described cytokine that emerges as a natural inhibitor of inflammatory and immune responses. However, IL-37 has not yet been investigated in bladder cancer, and its biological role is unknown. Objective: The purpose of this study was to investigate IL-37 serum levels in patients with bladder cancer and determine whether they were linked to the patients' pathological characteristics. Methods: IL-37 serum levels were measured using a commercial ELISA kit in 60 patients with transitional cell carcinoma (TCC) of the bladder (mean age: 64.55±12.93) and 50 healthy controls (mean age: 62.94±12.69). Non-parametric tests were used for statistical comparisons, and the Cohen's d effect size was calculated to evaluate the practical and clinical significance of the results. Results: Our findings indicated an increasing trend in IL-37 serum levels in patients with TCC (42.77±3.36 pg/ml) in comparison with controls (40.51±7.32 pg/ml, p=0.09). However, IL-37 serum levels were found to be significantly higher in male patients (44.72±3.81 pg/ml) and patients aged ≥70 (46.92±6.77 pg/ml) in comparison with male controls (29.96±3.30 pg/ml, p=0.026) and controls aged ≥70 (23.62±4.43 pg/ml, p=0.009). In comparison to similar controls, Cohen's d effect size for patients aged ≥70 years was found to be 0.90. Conclusion: The findings reveal a higher serum level of IL-37 in patients with TCC, which might be clinically associated with immunosuppression and tumor growth. However, this is a preliminary study, and more research on the biological role of IL-37 and its potential therapeutic effects in bladder cancer is required.