Increased recognition of Rhodococcus equi as a human pathogen has occurred since 1983, when the first review article summarized the world's literature of 12 cases. In this article, we present 12 cases from the University of Oklahoma Health Sciences Center and review 60 from the literature. Most cases occur in immunocompromised hosts and present as chronic cavitary pneumonias. Associated extrapulmonary disease is seen at diagnosis in 7% of patients with pneumonia, and relapse occurs at extrapulmonary sites in 13%, often without reappearance of pulmonary disease. Relapse may follow a course of antimicrobial therapy that is too brief, but can also occur during treatment. Infections also occur in the gastrointestinal tract, causing enteritis and regional adenitis with abscesses. Contaminated wounds may become infected. Isolated bacteremias may be a manifestation of latent infection recurring during a period of immune suppression. A common feature of human R. equi infection is delay in diagnosis. The insidious course of disease contributes to delay, as does failure to identify the organism. R. equi is easily cultured on nonselective media but commonly mistaken for a diphtheroid or occasionally for a mycobacterium based on acid-fast appearance. Form and duration of treatment are closely related to host immune status. Immunocompromised patients require prolonged or indefinite therapy with multiple antibiotics. Infections in immunocompetent hosts are easily treated with short courses of single agents. Infections related to contaminated wounds are treated primarily by irrigation and debridement. Infections in immunocompromised hosts are increasing in frequency largely due the AIDS epidemic. Infections in immunocompetent hosts, reported rarely before this series, may be underdiagnosed, perhaps because R. equi resembles common commensals and has limited virulence in this population. This report demonstrates that R. equi infections, including community-acquired pneumonias, occur in immunocompetent hosts.