Lung abscess caused by Rhodococcus equi.

In the immunocompromised patient, early diagnosis of a lung cavity is essential for appropriate treatment. Rhodococcus equi (formerly Corynebacterium equi) is a variably acid-fast bacterium that can produce cavitary disease in an immunocompromised host. The two cases presented here demonstrate the clinical and radiographic features of R equi lung abscess. The first patient was on long-term corticosteroid therapy for rheumatoid arthritis. The second patient had AIDS. The correct diagnosis in both cases was delayed because acid-fast bacilli seen on smears of sputum were presumed to be Mycobacterium tuberculosis.