Patients (12) with Wegener''s granulomatosis were treated with antimicrobial agents, chiefly trimethoprimsulfamethoxazole. The clinical course improved in 11 of the 12 patients who received this treatment. The success of antimicrobial treatment suggests the possibility of a microbial infection as the inciting cause of Wegener''s granulomatosis in some patients. Alternatively, these agemts im particular, trimethoprim-sulfamethoxazole may posses immumnosuppressant activity.