The overall prognosis of acute pyelonephritis is good, but the infections by extended spectrum Ղ-lactamase (ESBL) producing Escherichia coli (E.coli) cause poor responses to empirical antibiotic treatment, and consequently increase mortality. ESBL can hydrolyze the antibiotics with a Ղ-lactam ring and confer resistance to oxyimino-cephalosporins and monobactams. If the patient shows poor responses to empirical antibiotics or severe septic conditions, physicians must switch the antibiotics to other antibiotics covering resistant strains without delay. We report a case of acute pyelonephritis by extended-spectrum Ղ-lactamase producing E.coli in a 29-year-old woman who was empirically treated with oral ciprofloxacin as an initial treatment, but progressed to sepsis.