BACKGROUND: The toxicity of high-concentration carbon monoxide (CO) on human health has previously been documented. However, the epidemiological evidence on the association between acute exposure to ambient CO and respiratory diseases is relatively lacking and controversial. OBJECTIVES: To examine the short-term association between ambient CO and hospital outpatient visits for respiratory diseases in Dongguan, China. METHODS: The number of daily hospital outpatient visits for respiratory diseases, and air pollution and meteorological data were collected from January 2013 to August 2017. A generalized additive model with a quasi-Poisson link was used to estimate the association between ambient CO concentration and the total number of hospital outpatient visits for all respiratory diseases and those for asthma, bronchiectasis, chronic obstructive pulmonary disease (COPD) and pneumonia. We further analyzed the effect of ambient CO by gender and age. RESULTS: Over the study period, a 24-h mean concentration of ambient CO of 0.88 mg/m3 (below the limit for CO in China) and a total of 89,484 hospital outpatient visits for respiratory diseases were recorded. Ambient CO was found to increase the risk for asthma, bronchiectasis, pneumonia and the total number of respiratory diseases. The per interquartile range (IQR) increase in ambient CO at lag03 day corresponded to a 5.62% (95% confidence interval (CI): 3.24%, 8.05%), 8.86% (95% CI: 4.89%, 12.98%), 6.67% (95% CI: 0.87%, 12.81%) and 7.20% (95% CI: 2.35%, 12.29%) increased risk in outpatient visits for all respiratory diseases, asthma, bronchiectasis and pneumonia, respectively. Each association was partially weakened after adjusting for co-pollutants. The effect of ambient CO on respiratory diseases appeared to be greater for females and the elderly. CONCLUSIONS: Short-term exposure to ambient CO was associated with increased risk of outpatient visits for respiratory diseases. Our analysis may help to understand the health effects of low-levels of CO and provide evidence for the creation of air quality standards.