BACKGROUND: We conducted a study to determine factors associated with the occurrence of mediastinitis in patients hospitalized for cervical necrotizing fasciitis (CNF). METHODS: We reviewed the medical records of 130 consecutive patients in an intensive care unit (ICU) who were hospitalized with a diagnosis of CNF. Two radiologists reviewed cervical and thoracic computed tomography (CT) scans to determine the source and extension of the infection in each patient. RESULTS: Among the cohort of 130 patients with CNF, 37 (28%) had mediastinitis at the time of their admission (which in 13 cases was superior, or above the aortic arch, and in 24 cases inferior). Cervical necrotizing fasciitis complicated by mediastinitis resulted in a longer stay in the ICU than did CNF without mediastinitis, of a mean of 29 days (range, 18 to 39 days) versus 14 days (range, 9 to 19) days, respectively (p < 0.0001). Multivariate analysis revealed that the presence of mediastinitis was associated with oral intake of glucocorticoids before admission (odds ratio [OR], 2.17; range, 0.99 to 4.76), a pharyngeal focus of CNF (OR, 2.17; range, 1.04 to 4.53), or gas seen on an initial CT scan (OR, 4.49; range, 2.15 to 9.38). Both a pharyngeal focus of fasciitis and the presence of gas were strong independent predictors of inferior mediastinitis (OR, 15.1; range, 4.9 to 46.4; p < 0.0001). CONCLUSIONS: The present study is the first to describe three independent factors associated with extension of cervical fasciitis to the thoracic cavity, including glucocorticoid intake before admission, and confirms previous reports of a high incidence of mediastinitis in patients with CNF.