Between 1984 to 1991 55 children (mean age 7.0 years) were observed with a painful effusion of the hip examined by ultrasound. Children with radiologically diagnosed hip diseases were excluded. In 47 cases we found a transient synovitis of the hip which started at an average interval of 4 days in mean before admission. All children were reexamined 6 weeks after the end of treatment sonographically and clinically, 4 of those revealing incipient Perthes diseases. The primary sonographic right/left difference in amount of effusion (mean) was 4.0 +/- 1.7 mm and after aspiration 1.5 +/- 1.4 mm with a significantly correlation with the amount of aspirated synovial fluid. The radiological findings for a diagnosis of an effusion of the hip were not reliable. Beside the diagnostic signs of the appearance of the aspirated fluid the sonographic assisted aspiration of the hip is an important treatment mode to reduce the intracapsular pressure and the pain of a hip effusion in children.