Chronic inflammation is a well-known risk factor for type 2 diabetes mellitus (T2DM). The influence of chronic osteomyelitis (COM), an inflammatory disease, on the risk of developing T2DM remains unknown. This study investigated the risk of developing T2DM among COM patients. Using a retrospective cohort study, we identified 20,641 patients with COM and 82,564 age- and sex-matched controls for comparison from the Taiwan National Health Insurance Database (NHIRD) from 1997 to 2010. We followed up the COM cohort and the comparison cohort to compare the incidences of diabetes (ICD-9-CM code 250) until the end of 2010 or until the patients were censored because of death or withdrawal from the insurance program. The diabetes risk was analyzed using the Cox proportional hazards regression model. The incidence of T2DM was 1.6-fold higher in the group of COM patients than in the comparison group (29.1 vs. 18.2 per 10,000 person-years). The COM patients exhibited a higher diabetes risk [adjusted hazard ratio (aHR) = 1.64, 95 % confidence interval (CI) = 1.44-1.87] after controlling for the baseline and comorbidities. Younger and higher income patients exhibited a higher COM-to-reference incidence rate ratio (IRR) for T2DM compared with that of their counterparts. We also observed an increased risk of T2DM in COM patients with comorbidities (aHR = 1.70, 95 % CI = 1.47-1.96) compared with that of their non-COM counterparts. This is the first study to report the association between COM and an increased risk of developing T2DM, particularly among younger and higher income patients.