Brain infections are generally well visualized by contrast-enhanced computed tomography or magnetic resonance imaging. However, these modalities are often unrevealing in the encephalopathy produced by the human immunodeficiency virus and in the early stages of herpes simplex encephalitis. Several studies have documented the greater sensitivity of perfusion single-photon emission computed tomography (SPECT) in human immunodeficiency virus encephalopathy. In herpes simplex encephalitis, a few case reports have documented that SPECT may depict increased perfusion in the characteristically involved temporal and other limbic structures when computed tomography and magnetic resonance imaging are normal.