The ultrasonic findings in 38 children with osteomyelitis of the limb bones were analysed in four time-related groups based on the interval between the onset of symptoms and the ultrasonic examination. Deep soft-tissue swelling was the earliest sign of acute osteomyelitis; in the next stage there was periosteal elevation and a thin layer of subperiosteal fluid, and in some cases this progressed to form a subperiosteal abscess. The later stages were characterised by cortical erosion, which was commonly present in those who had had symptoms for more than a week. Concurrent septic arthritis was revealed in 11 patients, most frequently in association with osteomyelitis of the proximal femur or the distal humerus. Four weeks after clinical cure, ultrasonic examination showed no abnormalities. Ultrasonography is therefore a useful additional method for the diagnosis and assessment of osteomyelitis and its complications.