PURPOSE: To analyze the related sonographic findings and to determine the value of sonography in establishing the diagnosis of acute osteomyelitis in infants. MATERIALS AND METHODS: The sonographic findings of eleven infants aged 10 days-4 months (mean, 45 days) with acute osteomyelitis were retrospectively evaluated. The involved bones were the femur (n=5), humerus (n=2), tibia (n=2), rib(n=1), sternum(n=1), and calcaneus(n=1). Discontinuity or destruction of cortical margins, echotexture of the metaphysis and epiphysis, the presence of subperiosteal hypoechoic lesion, adjacent soft tissue swelling, distension of the joint capsule, the echotexture of joint effusion, and dislocation or subluxation of the involved joint were evaluated. The sonographic findings were compared with the plain radiographic (n=12) and MR (n=5) findings, with special attention to the identification of the metaphyseal or epiphyseal bony lesions and the involvement of adjacent joints. RESULTS: The sonographic findings of osteomyelitis were cortical discontinuity or destruction (n=12), hypoechoic lesions with an echogenic rim in the metaphysis (n=12), subperiosteal hypoechoic lesions (n=8), soft tissue swelling (n=9), a distended hip joint, with echogenic fluid (n=5), ill-demarcated echogenic lesions in the capital femoral epiphysis (n=5), and a subluxated hip joint (n=3). Plain radiographs revealed well or ill-defined osteolytic lesions in the metaphysis, accompanied by cortical destruction (n=8), new periosteal bone formation (n=3) and reactive sclerosis (n=2). Abnormality of the femoral epiphyses and joint involvement were not detected on plain radiographs, and in four cases no abnormality was noted. MR imaging showed that at T1WI, affected bony lesions were of low signal intensity and enhanced, with high signal intensity at T2WI. In all cases, both metaphyseal and epiphyseal lesions were demonstrated at MRI, but in one of the three cases in which an epiphyseal lesion was seen at MRI, this was not detected at US. CONCLUSION: Sonography is not only more sensitive than radiography in evaluating metaphyseal bony lesions but also useful in assessing concomitant joint and epiphyseal involvement of acute osteomyelitis in infants. Sonography is, therefore, a useful additional diagnostic tool for the early detection and management of acute osteomyelitis in infants.