Introduction:We sought to describe focal bone lesions detected on magnetic resonance imaging (MRI) in patients following ultrasound diathermy. Methods: We performed a retrospective analysis of MRI characteristics of bone lesions in 10 patients who underwent ultrasound diathermy for musculoskeletal conditions followed by plain radiographs and MRI for pain. Magnetic resonance arthrography was performed in two patients and one patient had a second follow-up MRI examination. A phone interview was conducted with all patients. Results: All 10 patients had experienced mild to severe pain during ultrasound diathermy, with four (40%) prematurely terminating the treatment session as a result. Plain radiographs were normal. On MRI, a total of 12 lesions were observed in the 10 patients, with nine lesions in eight shoulder joints, two lesions in one ankle joint, and one lesion in the first metacarpal base. All lesions were small to medium-sized (6 × 5 × 3 mm to 32 × 23 × 9 mm), well-demarcated and subcortical in location with a crescentic or oblong configuration. Lesions were highly conspicuous on T2-weighted images and relatively inconspicuous on T1-or intermediate weighted sequences. Typically, larger lesions had a T2-hyperintense rim surrounding an isointense central area. Leakage of arthrographic contrast into the lesion was observed. The overall features favoured focal osteonecrosis in all cases. The affected locations typically had little overlying subcutaneous tissue, were close to tendon insertions, and corresponded to the site of ultrasound diathermy application in all patients. Conclusion: Appearances akin to focal osteonecrosis may result from application of ultrasound diathermy. Recognition of the characteristic appearance will allow this connection to be made to avoid subsequent unnecessary workup.