72 patients were studied by ultrasonography aimed at demonstrating its usefulness in the diagnosis of the irritable hip syndrome in children. a real time equipment with sectorial transductor of 5 mhz was used to compare both hips. effusion was found in 43 patients. the articular effusion predominated only among those patients with transient synovitis, whereas capsular thickening with alterations of the head of femur prevailed in perthes’ disease. a predominance of an anecoic effusion with small ecogenic images was observed in the case of septic arthritis. ultrasonography was considered as an efficient elective method to make differential diagnoses of the diseases grouped within the irritable hip syndrome.