Background: Urinary tract infections are the most common bacterial infections during pregnancy. Although asymptomatic bacteriuria is the most prevalent form of infection, acute pyelonephritis is the most serious complication, which occurs in 1% -2% of all pregnancies as the leading cause of septic shock in pregnancy and related to increased risk of cerebral palsy. Objectives: This study was performed regarding various results of previous studies and lack of a similar study to evaluate the prevalence of acute pyelonephritis and its outcomes in pregnant women admitted to Taleghani hospital, Tabriz, Iran. Patients and Methods: All pregnant women admitted with acute pyelonephritis from March 2010 to October 2011 to Taleghani hospital, were enrolled in this study. Antibiotic therapy, urine cultures one or two weeks after treatment and late consequences such as pregnancy outcome, newborn birth weight and Apgar score were evaluated. Results: The mean age was 23 ± 4.88 years. Of 60 patients, 40 (66%) were nulliparous and 19 (31.7%) multiparous. The mean gestational age was 25.73 ± 7.42 weeks. Most of patients (65%) had been diagnosed in the second trimester of pregnancy. Regarding urine culture studies, 42 (70%) had positive and 18 (30%) negative results. The predominant pathogen was E. coli isolated from 33 (78.6%) cultures, Klebsiella from 8 (19%) and negative coagulase staphylococcus from one (2.4%). None of the infants had low birth weight and 4 of them had preterm delivery (less than 37 weeks). The frequency of acute pyelonephritis was 0.46% in total deliveries. Conclusions: Similar to previous studies, most cases diagnosed in the second trimester of gestation, which necessitates more investigation in this field, since timely diagnosis and treatment have a significant role in preventing known complications of the disease.