Objective:To investigate the prevalence of small intestinal bacterial overgrowth (SIBO) in patients with rosacea, and to analyze the relationship between breath test results and the occurrence of rosacea.Methods:Patients with rosacea were enrolled from the outpatient department of Xiangya Hospital from March 2022 to June 2023. The methane-hydrogen breath test was used to detect intestinal levels of methane and hydrogen in all patients to investigate the prevalence of SIBO. The basic information, clinical symptoms and severity, quality of life scores, gastrointestinal symptoms, and past medical history of the patients were collected. Statistical analysis was carried out by using the chi-square test, nonparametric test and multivariate logistic regression models to investigate the relationship between SIBO and the occurrence of rosacea.Results:A total of 116 patients with rosacea completed the methane-hydrogen breath test. They were aged 18 to 56 years (median [ Q1, Q3]: 25 [22, 33] years), and included 7 males (6.0%) and 109 females (94.0%) ; there were 43 cases (37.1%) of erythematotelangiectatic rosacea, and 73 (62.9%) of papulopustular rosacea. As the breath test showed, 94 patients were diagnosed with SIBO (81.0%, 95% CI: 72.7% - 87.7%) based on the breath tests, 84 showed positive hydrogen breath test results (72.4%, 95% CI: 63.3% - 80.3%), and 47 had positive methane breath test results (40.5%, 95% CI: 31.5% - 50%). Among the 67 patients with moderate to severe erythema, 33 (49.3%) showed positive methane breath test results, and 14 of 49 (28.6%) patients with mild erythema showed positive methane breath test results, with a rate difference of 20.7% ( P = 0.025, 95% CI: 13.9% - 27.5%) ; there were no significant differences in the positive rates of SIBO and hydrogen breath test results between the patients with moderate to severe erythema and those with mild erythema (both P > 0.05). No significant differences were observed in the age, gender, clinical subtypes, severity of papulopustules, flushing and burning sensation, or rosacea quality of life index scores between the SIBO-positive and -negative groups, between hydrogen-positive and -negative groups, and between methane-positive and -negative groups (all P > 0.05). Multivariate logistic regression analysis showed that methane positivity on breath test was associated with the severity of erythema in rosacea ( OR = 2.495, 95% CI: 1.102 - 5.649, P < 0.05) . Conclusions:The prevalence of SIBO was relatively high in the patients with rosacea. However, only the positive rate of methane breath test differed between the rosacea patients and non-rosacea controls, and there was some correlation between methane positivity on breath test and increased severity of rosacea erythema.