We studied the frequency and cause of a feature exhibited on T1-weighted (T1-W) magnetic resonance (MR) imaging termed the 'penumbra sign' in a series of patients presenting with osteomyelitis and correlated it with the double-line sign described as a T2-weighted (T2-W) or short tau inversion recovery (STIR) feature of both the Brodie's abscess and avascular necrosis. The clinical, radiographic, MR imaging, microbiological and histological findings in 32 patients referred to an orthopaedic oncology service, but subsequently proven to have osteomyelitis, were reviewed. The presence or absence of a rim of tissue lining an abscess cavity typified by minor signal hyperintensity relative to the main abscess contents on T1-W MR imaging (the 'penumbra sign') was identified. The sign was correlated with the radiographic and other findings. The penumbra sign was identified in 24 cases (75%) and appears to be a more sensitive sign than the corresponding double-line sign which was evident in only 29% of these on T2-W or fast STIR images. The lesions were unilocular in 11 cases (46%) and multilocular in 13 (54%). The thickness of the penumbra ranged from 2 to 5mm. On histological examination the tissue comprising the penumbra sign was found to be highly vascularized granulation tissue containing thick walled arterioles. The penumbra sign is characteristically seen on T1-W MR images in subacute osteomyelitis and is due to a thick layer of highly vascularized granulation tissue which may not be visible as the double-line sign on T2-W or fast STIR sequences. This characteristic, but not pathognomonic, MR finding supports the diagnosis of bone infection and helps to exclude the presence of a tumour.