Although significant differences exist in the methods of treatment and prognoses of septic coxitis and transient synovitis in children complaining of acute hip pain, similar symptoms are present in these two diseases at the early stages, and differential diagnosis is difficult. To differentiate between these two diseases, the authors evaluated the clinical, serologic, and radiologic findings and tried to determine factors that could be used as diagnostic criteria. The authors performed a retrospective study by evaluating medical records, plain hip radiographs, and clinical findings in 97 patients with transient synovitis and 27 patients with septic arthritis. Univariate analysis showed significant differences in body temperature, serum WBC count, and ESR and CRP levels of the two patient groups. Plain radiographs showed a displacement or blurring of periarticular fat pads in all patients with acute septic arthritis, and multivariate regression analysis showed that body temperature >37 degrees C, ESR >20 mm/h, CRP >1 mg/dL, WBC >11,000/mL, and an increased hip joint space of >2 mm were independent multivariate predictors of acute septic arthritis. The authors conclude that the independent multivariate predictors are effective indices for the differential diagnosis of acute septic coxitis and transient synovitis.