Moxifloxacin is an oral 8-methoxyquinolone with a broad spectrum of anti-microbial activity. Clinical trials indicate that moxifloxacin is a safe and well-tolerated antibiotic. We describe a potentially serious interaction between moxifloxacin and warfarin in three frail, elderly patients. The healthy volunteers studied for the moxifloxacin-warfarin interaction differ from these patients.