INTRODUCTION AND HYPOTHESIS: Interstitial cystitis/bladder pain syndrome (IC/BPS) and irritable bowel syndrome (IBS) often occur concomitantly without an obvious reason. It is important to determine the relationship between these related diseases. We aimed to determine whether IBS increase the risk of IC/BPS. METHODS: We identified newly diagnosed IBS patients between 2002 and 2013 from a nationwide database as the IBS cohort. Subjects diagnosed with IC/BPS before IBS were excluded. Cox's regression analysis with a hazard ratio (HR) of IC/BPS between IBS and the non-IBS cohort was applied to unmatched and matched (16 confounders of propensity scores) models. The time from diagnosis of IBS to IC/BPS was also calculated. RESULTS: In the unmatched group, which included 100,124 IBS (55% female) and 874,048 non-IBS patients, the IC/BPS adjusted HR was 1.292 (95% confidence interval [CI], 1.131-1.476;p < 0.0001) in the IBS cohort compared with the non-IBS cohort. In the matched group, there were 85,359 patients in each cohort, and the IC/BPS HR was 1.599 (95% CI, 1.344-1.903; p < 0.0001). The average numbers of years until the development of IC/BPS in the IBS cohort and non-IBS cohort were 4.60 ± 2.58 (n = 253) and 5.99 ± 3.49 (n = 295) years, respectively. CONCLUSIONS: IBS was shown to increase the risk of IC/BPS in this 12-year cohort study. The time from the diagnosis of IBS to IC/BPS was 5.35 ± 3.18 years. A common pathophysiology of IBS and IC/BPS is possible. Clinicians should be mindful of the association and promote collaborative care of these two elusive diseases.