Neurologic involvement is common in patients with acquired immune deficiency syndrome (AIDS). To assess the computed tomographic findings in AIDS, the authors reviewed 12 consecutive patients found to have focal brain lesions representing AIDS complications on computed tomography and having histologic or clinical proof of the diagnosis. Five patients had toxoplasmosis, six had lymphoma and one had both conditions sequentially. The finding of multiple lesions and mass effects with edema was more common with toxoplasmosis. In six patients who had toxoplasmosis 33 nodules were observed compared with only 8 in seven patients who had lymphoma. Disease extending linearly along the ventricular wall was typically seen in patients who had lymphoma.