Children with a painful hip present a diagnostic challenge since clinical differentiation between septic arthritis, transient synovitis and Perthes disease may be difficult. Septic arthritis, a potentially life-threatening and debilitating medical emergency, requires early recognition for successful treatment, while transient synovitis and Perthes disease may be managed conservatively. An "ideal" single test for discrimination between these conditions is currently not available. We assessed the value of clinical examination and simple laboratory tests together with radiography and hip ultrasound in differentiating septic arthritis from transient synovitis and Perthes disease by analyzing the records of 89 children treated at our institution for hip pain. Ultrasound, radiographs, laboratory, clinical, and follow-up data were available for all the children. Diagnoses were made according to established criteria. Transient synovitis was present in 64 patients, septic arthritis in 8 (of whom 2 had additional osteomyelitis), and Perthes disease in 4. All children with septic arthritis had hip effusion shown by ultrasound and at least two of the following criteria: fever, elevation of erythrocyte sedimentation rate (ESR) and of C-reactive protein (CRP). None of the children without effusion on ultrasound or who lacked two or all criteria had septic arthritis. Radiographs had no significant impact on the decision-making in primary evaluation of acute hip pain. CONCLUSION: We conclude that investigation of painful hips in children, based on hip ultrasound, body temperature, ESR and CRP, may allow cases for hip joint aspiration to be selected efficiently and may reduce the number of radiographs and hospital admissions.