Nocardiosis is an illness caused by several species of the genus Nocardia. Although it is seen in immunocompromised hosts, infection frequently occurs in persons with no recognized predisposition. Infection may be localized to the skin, or it may involve the lungs, disseminating to virtually any organ. Pathogenesis is only beginning to be understood, but, at least in laboratory animals, there seems to be a host-parasite relationship that depends on inadequate clearing of the organism by processes of cell-mediated immunity. Clinical manifestation varies widely according to the sites involved. Diagnosis is principally by culture and may require an aggressive approach. Although sulfonamides remain the mainstay of therapy, preliminary data for other antimicrobial agents are promising. © 1980, American Medical Association. All rights reserved.