We report four human immunodeficiency virus (HIV)-infected patients (3 men and one woman, average age, 34.3 years) with pulmonary infection (two with pneumonia and two with lung abscess) caused by Rhodococcus equi. These patients, who presented with fever and productive cough, were admitted to Nakornping Hospital in northern Thailand. Chest roentgenograms showed pulmonary infiltration and/or cavitary lesions. Their conditions were poor because of severe anemia, and transfusion was necessary in three of the four patients. Before culture results were available, the etiologic microorganisms identified in sputum smears were gram-positive and acid-fast coccobacilli. One of the four patients had a mixed infection with R. equi and Salmonella enteritidis. The mean CD4 lymphocyte count in the three tested patients was 10/mm3 (CD4/CD8 ratio = 0.057). Four isolates of R. equi were sensitive to imipenem, minocycline, erythromycin, vancomycin, and ciprofloxacin (minimum inhibitory concentrations; MICs, <or=1.56 microg/ml), but resistant to most beta-lactam antibiotics. Two isolates were sensitive (MICs, 0.20 and 0.78 microg/ml) and two resistant (MICs 50 and >100 microg/ml) to rifampicin. Two patients were treated with erythromycin plus rifampicin, while the other two were treated with anti-tuberculous drugs. However, treatment was ineffective; three patients subsequently died because of respiratory failure, and one patient did not improve and was transferred to another hospital in her hometown.