Ten cases of acute suppurative thyroiditis were studied. All patients remained euthyroid clinically during the course of this disease and during the follow-up. Of seven cases in which bacterial cultures were performed, an aerobic bacterium alone was isolated in one patient and anaerobic bacteria alone in four patients. Mixed aerobic and anaerobic microorganisms were recovered from one patient and cultures were negative in one case. Plasma TSH, T4, T3, and PBI levels were normal except in one case. The 131I uptake values were normal in six of seven cases. Plasma TSH response to TRH stimulation was normal in four of five cases. Antibodies against thyroglobulin and thyroid microsome were negative in four of six cases. These findings suggest that anaerobic bacteria may play an important role in the pathogenic process of acute suppurative thyroiditis and that the pituitary-thyroid functions are not impaired.