OBJECTIVES: Pneumonia is one of the main causes of readmissions in the United States, with estimates of 30-day readmissions as high as 20%. Given the high costs associated with readmissions, it is important to understand what factors contribute to readmissions in this patient population. Hospital volume, an indicator of availability of resources and staff, may be associated with readmission rates. In this analysis of a large, single-network hospital system, we examined the association between hospital volume and 30-day readmission in patients with pneumonia. METHODS:  Adult (18 or older) patients admitted to one of the 11 Advocate Health Care hospitals with a pneumonia diagnosis between 2008 and 2015 were included for analysis. Hospitals were categorized as low, medium, or high volume based on inpatient beds. 30-day readmission was compared by volume and by patient gender, race, age, length of stay, insurance, calendar year, and discharge disposition (home or home health). Logistic regression was used to examine the association between hospital volume and 30-day readmission, adjusting for covariates. RESULTS:  37,757 pneumonia admissions occurred between 2008 and 2015. 30-day readmission rates were 10.5%, 11.5%, and 12.6% for low-, medium-, and high-volume hospitals, respectively. In adjusted models, increased odds of 30-day readmission were seen for patients in both medium-volume (OR = 1.14; 95%CI 1.04-1.25) and high-volume hospitals (OR = 1.28; 95%CI 1.23-1.34) when compared with low-volume hospitals. CONCLUSIONS: In this analysis of a single hospital network, patients treated for pneumonia in higher-volume hospitals were found to have significantly greater odds of 30-day readmission than in lower-volume hospitals, even after adjustment for other risk factors. This finding is surprising and should be investigated further to better understand the reasons for these differences.