To study the microbiologic and clinical characteristics of patients with mediastinitis.Retrospective review of clinical and laboratory data of 17 patients treated between 1980 and 1987.Aerobic or facultative bacteria only were present in three patients (18%), anaerobic bacteria only in seven (41%), and mixed aerobic-anaerobic flora in seven (41%). In total, there were 42 isolates, 13 aerobic or facultative and 29 anaerobic bacteria, an average of 2.5 per specimen. Anaerobic bacteria predominated in infections that originated from esophageal perforation and orofacial, odontogenic, and gunshot sources. The predominant aerobes were alpha-hemolytic Streptococcus (three isolates), Staphylococcus aureus (two isolates), and Klebsiella pneumoniae (two isolates). The predominant anaerobes were Prevotella and Porphyromonas species (eight isolates), Peptostreptococcus species (seven isolates), and Bacteroides fragilis group (three isolates).These data highlight the polymicrobial aerobic-anaerobic nature of mediastinitis.