To quantify the risk of nocardiosis in various populations, I systematically reviewed articles published between 1966 and 2004. The incidence of nocardiosis in 3 large, geographically defined populations ranged from 0.35 to 0.4 cases per 105 persons·year. In contrast, the incidence of nocardiosis among people with human immunodeficiency virus (HIV) infection or acquired immunodeficiency syndrome (AIDS) in 1 study was 53 nocardiosis cases per 105 persons·year, approximately 140 times greater than that in the geographically defined populations. The frequency of nocardiosis cases in 4 populations of HIV-infected people averaged 608 cases per 10 5 persons. The incidence of nocardiosis in bone marrow-transplant recipients at 1 hospital was 128 cases per 105 persons·year, an incidence approximately 340 times greater than that in the geographically defined populations and in the same range as in HIV-infected people. The frequency of nocardiosis in 21 series of cases in recipients of a variety of transplanted organs averaged 1122 cases per 105 persons. These estimated incidence rates are imprecise because they were not collected through prospective surveillance systems, but the estimates for the 3 groups were internally consistent and provide useful information for clinicians.