Rhodococcus equi is a facultative intracellular, obligate aerobe, partially acid fast, gram-positive pathogen that causes cavitary pneumonia in animals and immunocompromised humans. We describe 8 cases of R. equi pneumonia in patients with advanced HIV infection (CD4 counts less than 100/mm3), 7 males and 1 female (mean age 30.8 years), observed between 1991 and 1994. A history of exposure to farm animals was found in 4 patients. The most common presenting symptoms were fever, malaise, dyspnea, cough and hemoptysis, chest pain and weight loss. Chest x-rays showed tipical focal area of consolidation throughout the lung (3 upper, 3 lower and 2 middle fields) associated with cavitation in 4 cases. The definitive diagnosis in our hands was delayed only in the first case in which conflicting data resulted from blood culture (Bacillus sp. isolation) and sputum examen (acid-fast bacterium in the Ziehl-Neelsen stain). Final microbiological diagnosis depended on blood cultures (n=5), bronchoalveolar lavage (n=1), sputum (n=1), lung biopsy (n=1). All the patients were treated with prolonged courses of antibiotic therapy (259 days, range 120-340 in 6 dead patients; more than one year and two months respectively in two patients alive). According to microbial susceptibility TMP/SMX, vancomycin, imipenem, rifampin, aminoglycosides, macrolides and quinolons were more frequently used. Resistant R. equi mutants were selected during therapy with TMP/SMX (n=2), rifampin (n=1) and erythromycin (n=1). Five patient underwent pulmonary lobectomy after exclusion of metastatic bacterial lesions. Only 2 patients are alive, one after 365 days of antibiotic therapy and upper lung lobectomy, one after 60 days of antibiotic therapy. Optimal antimicrobial therapy and the role of surgery remain, in our experience, uncertain.