Acute osteomyelitis in childhood is uncommon. Early diagnosis is often difficult. We present a 7-year-old boy with clinical manifestation of fever and progressive left thigh swelling for one week. Sonography of the left thigh showed periosteal thickening with subperiosteal abscess around the metaphysis of the left distal femur. Tc-99m bone scintigraphy showed increased uptakes at the left distal femur. Osteomyelitis was suspected from the examination of ultrasound and bone scan examinations. His symptoms were not relieved following systemic antibiotic therapy and a follow-up color Doppler sonography showed vascular flows within and around the periosteum indicating active inflammation. He finally received surgical drainage and the operative findings confirmed the diagnosis of osteomyelitis. In conclusion, ultrasound may be a helpful and convenient tool for establishing the early diagnosis of pediatric osteomyelitis. Ultrasound and color Doppler sonography can be used as clinical parameters to evaluate the disease severity and the progression of osteomyelitis in children.