An eighteen year young adult presented with painless swelling over left elbow joint of 4 months duration in absence of constitutional symptoms. Range of motion atelbow joint was painful but  preserved  . Radiographic evaluation showed cortical erosion with lytic lesion in left olecranon. Magnetic resonance Image showed altered signal intensity; hypo on T1 and hyper– intense on T2 in the left olecranon. Open biopsy showed epitheloid granuloma and Langrhans giant cells. Polymerase chain reaction identified atypical mycobacterium DNA. The patient received 6 months of chemotherapy. Lesion showed progressive resolution on serial radiograph with good function outcome. It requires high index of suspicion and prompt evaluation for early diagnosis. To reach final diagnosis histopathological examination should be performed in addition to nucleic acid amplification/PCR. Institution of timely chemotherapy decreases the morbidity and preserves good joint function, avoids further complications.