To compare the performance of computed tomography (CT) and magnetic resonance (MR) imaging in diagnosis of osteoid osteoma. Nineteen patients with histologically proved osteoid osteoma underwent CT and MR imaging before excision of the lesion. CT and MR images were compared regarding lesion conspicuity and detection of marrow, soft-tissue, and/or synovial changes adjacent to the primary lesion. CT was more accurate than MR imaging in detection of the osteoid osteoma nidus in 63% of cases. MR imaging was better than CT in showing intramedullary and soft-tissue changes in all cases. This may produce a misleading aggressive appearance on MR images. There was a statistically significant correlation between presence or absence of marrow or soft-tissue changes and treatment with antiinflammatory medications (P < .05). CT remains the best imaging modality for diagnosis of osteoid osteoma. MR images should not be interpreted without reference to plain radiographs and CT scans if serious errors in diagnosis are to be avoided.