We present descriptions of two human Mycobacterium microti infections: one of a patient with pulmonary disease and one of a patient with osteomyelitis of the hip. Both patients had acid-fast bacilli and a positive Mycobacterium tuberculosis complex PCR from clinical specimens, but mycobacterial cultures remained negative. The microbiological diagnosis was established by molecular methods. CASE REPORTSPatient 1 was a 60-year-old human immunodeficiency virusnegative man who was admitted for a diagnostic pulmonary biopsy. One year prior to admission, the patient suffered from a 1-month period of weight loss, high fever, and night sweats. At that time, a chest X ray showed an infiltrate in his right lung and he was treated with a 6-month course of amoxicillin for a presumed pulmonary actinomycosis, which was diagnosed by cytological examination of a bronchoalveolar lavage sample. The infiltrate decreased during therapy and cleared 6 months after therapy. During follow-up, 6 months after cessation of amoxicillin, he developed a new lesion in his left upper lobe suspected of malignancy, with increased 18-fluordeoxyglucose uptake on a positron emission tomography scan. The lesion was resected by thoracotomy for diagnostic purposes. Histopathologic examination revealed granulomatous necrotic inflammatory tissue with acid-fast bacilli (AFB); malignant cells were not found. A Mycobacterium tuberculosis (MTB) complex PCR targeting the IS6110 sequence was positive (10). The lung tissue specimen was cultured in liquid medium (Bactec MGIT; Becton Dickinson [BD], Sparks, MD) and on agar-based media (Middlebrook and Cohn 7H10; BD) and egg-based media (Coletsos plus ossein agar; Bio-Rad, Marnes-la-Coquette, France). All cultures were incubated at 37°C for 8 weeks. After these cultures remained negative, a second attempt at mycobacterial culture was performed on tissue specimens stored at Ϫ20°C using the same media and 9 weeks of incubation. Again, mycobacterial cultures remained negative. Following the positive AFB staining and PCR results from the lung tissue specimens, sputum samples were collected on three separate occasions that were negative for AFB and mycobacterial culture. The patient was treated with isoniazid, rifampin, and ethambutol for a total of 6 months and pyrazinamide for the initial 2 months. One month after completion of therapy, the patient was well and without symptoms.Patient 2 was a 58-year-old man who was admitted for a total hip prosthesis because of avascular necrosis of his right hip. Nineteen months earlier, corticosteroid treatment was started for polymyalgia rheumatica. He developed pain in his right hip and night sweats 7 months before admission and 3 months after cessation of corticosteroids. He reported 30 kg of weight loss since the onset of pain in the hip. He had no pulmonary complaints and no abnormalities on chest X-ray examination.At surgery, a femoral head resection was performed, but no prosthesis was implanted since an infection was suspected intraoperatively. A Girdlestone situation was...

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