Objectives: Acute pyelonephritis is a common infectious disease, and in most cases, it is a mild disease that does not cause complications, but a serious disease that can lead to death. Several strains that cause acute pyelonephritis are already well known from previous studies. With the increase in the use of antibiotics, antibiotic-resistant strains are rapidly increasing, which makes treatment difficult. This study intends to describe the trend of acute pyelonephritis and changes in antibiotic-resistant strains. Methods: From January 2011 to August 2019, this study was conducted with patients diagnosed with acute pyelonephritis and treated at a tertiary medical institution. Adults over 18 years of age were included, and outpatient treatment, pregnant women, and transplant patients were excluded.  Results: A total of 2818 people were enrolled, of which 1631 (57.9%) were culture-positive. Escherichia coli(E. coli) was the most common strain, followed by Klebsiella, enterococcus, and pseudomonas. ESBL-positive strains corresponded to 19.6% (553 patients) of all patients. The incidence of acute pyelonephritis at the end of the study was 2.7 times higher than at the start of the study, and the ESBL positive E. coli culture rate also increased exponentially from 4.9% to 20.9%. In the case of the ESBL-positive strain, the number of readmissions was statistically significantly longer than that of the non-ESBL-positive group (1.7±1.3 vs 1.2±0.6, p < 0.001), and the period of antibiotic use was also significantly longer (12.3±2.2 vs. 5.4±5.2, p < 0.001). Conclusions: As the use of antibiotics increased, the number of antibiotic-resistant strains increased, and accordingly, the length of hospitalization, the number of readmissions, and the period of use of antibiotics significantly increased. It will be necessary to improve the patient's prognosis and prevent antibiotic resistance by identifying antibiotic-resistant strains and using antibiotics appropriately according to the degree of antibiotic resistance.