Magnetic resonance (MR) imaging was used to assess for the presence of bacterial myositis, rare outside the tropics, in 13 patients with either the acquired immunodeficiency syndrome (AIDS) (n = 11) or positive results of serologic tests for the human immunodeficiency virus but without other evidence of AIDS (n = 2). Bacterial myositis was diagnosed in six patients: in five it was caused by pyogenic bacteria, and in the other, by Mycobacterium tuberculosis; in each patient, little or no subcutaneous tissue alteration occurred. On T1-weighted images in three patients, muscle abscesses showed a rim of increased signal intensity corresponding to margins between drainable pus and edematous muscle. Subcutaneous tissues appeared normal in patients with bacterial myositis but was not in the others, in whom muscle abnormalities tended to be less prominent. The latter group included patients with lymphoma (n = 1), Kaposi sarcoma (n = 2), and carbunculosis (n = 1), and three patients in whom no diagnosis was made; lymphedema was presumed to account for imaging abnormalities in four of the latter group.