PURPOSE: To describe the radiographic and MR characteristics of subacute diaphyseal osteomyelitis of the long bone. MATERIALS AND METHODS: We retrospectively analyzed plain radiographs and MR images in nine children and two young adults with surgically-proven diaphyseal osteomyelitis of the long bones. MR imaging was carried out before and after Gd-DPTA administration. Patients charts were reviewed for clinical and laboratory data and histologic correlation was available in all cases. RESULTS: All patients had a history of a minimum of 2 weeks' duration. All presented with an insidious onset of mild pain; they showed apyrexia and normal erythrocyte sedimentation rate. In all patients, plain radiographs showed single or multiple linear periosteal reaction with cortical irregularity. Medullary abnormality was not found except in one case which showed an abscess cavity. MR images revealed thickening or irregularity of the cortex, periosteal reaction, bone marrow signal alteration, and perilesional edema. CONCLUSION: Subacute diaphyseal osteomyelitis of the long bone is broadly based on the cortex, and elicits more sclerosis and periosteal reaction. Familarity with plain radiographs and MR features may be useful in the diagnosis of subacute diaphyseal osteomyelitis.