Background: Upper urinary tract infection (UTI) or pyelonephritismay increase the pathogenesis rate and risk of severe complications in children due to the atrophy of the kidneys.Objective: A set of clinical symptoms, laboratory markers, and ultrasound findings were assessed to reach early diagnosis and prognosis of pyelonephritis in hospitalized pediatrics.Methods: A cross-sectional study was conducted on 104 Iranian children (95 girls and 9 boys) aged 1 month to 12 years with acute pyelonephritis during 2012-2018. The ultrasound examination of kidneys and urinary tract during the hospitalization period, the incidence of clinical symptoms, and the monitoring of laboratory markersin blood and urine were studied to identify the best predictive factors of early diagnosis of this bacterial infection.Results: Three-fourth of the patients had one of the four clinical symptoms of abdominal pain, constipation, dysuria, andvomiting, whileotherswere asymptomatic. A considerable frequency of pyuria(88.4%), Escherichia coli in urine (92%), leukocytosis (81.7%), and high ESR (> mm/h, 80%) and CRP (> 10mg/L, 82.8%) was observed. The kidney and urinary tract ultrasonography onlyin 32.7% of children showed in favor of pyelonephritis (cystitis, ureteral stones, and hydronephrosis).Conclusion: In the majority of patients, there was a high prevalence of clinical signs and laboratory markers associated with pyelonephritis. Since most patients have normal sonography, ultrasound alonecould notbe an efficient tool fortracingfebrile UTI.