OBJECTIVE We know that diabetes predisposes to common infections, such as cellulitis and pneumonia. However, the correlation between the level of glycemic control and the rate of infection is unknown. RESEARCH DESIGN AND METHODS We examined the association between glycemic control in patients with diabetes and the incidence of infection in the entire population of patients with diabetes in a large HMO. During the study period, we first selected an HbA1c test for each patient and then searched for an infection diagnosis in the 60 days that followed the test. A multivariate logistic regression analysis was performed to determine the independent effect of HbA1c on the likelihood of being diagnosed with an infection. We were able to control for many confounders, such as other chronic illness, time since the diagnosis of diabetes, and use of steroids before the infection. RESULTS We identified 407 cases of cellulitis. Multivariate logistic regressions for cellulitis showed a 1.4-fold increased risk among patients with HbA1c >7.5% (58 mmol/mol). Factors such as obesity, Parkinson’s disease, peripheral vascular disease, and prior treatment with prednisone predisposed to cellulitis. There was an increase of 12% in the odds of cellulitis for every 1% (11 mmol/mol) elevation in HbA1c (odds ratio [OR] 1.12; CI 1.05–1.19). A similar analysis showed a trend toward an increased risk of pneumonia in patients with HbA1c >7.5% (58 mmol/mol) (OR 1.1; CI 0.9–1.4). CONCLUSIONS Poor glycemic control was associated in this study with the development of cellulitis. The study also suggests that exposure to oral prednisolone increases the risk of cellulitis, pneumonia, and upper respiratory infection.