Actinomycosis is a rare infectious bacterial disease typically involving cervicofacial, abdominopelvic, or thoracic regions, caused by Actinomyces species. However, involvement of the mediastinum as the primary site of origin is extremely rare. An elderly patient complained of left‐sided chest pain. The chest radiograph revealed pleural effusion, which revealed lymphocyte‐predominant exudates. Chest computed tomography showed a soft tissue mass in the left anterior mediastinum. Positron emission tomography revealed an increased uptake of F‐fluorodeoxyglucose in this lesion. To exclude malignancy and make a confirmative diagnosis, a mediastinal biopsy was performed through video‐assisted thoracic surgery. Finally, mediastinal actinomycosis was diagnosed. We report herein a case of mediastinal actinomycosis mimicking lung malignancy presented with recurrent lymphocyte‐predominant pleural fluid exudate. In patients with a recurrent or unexplained exudative pleural effusion, it may be worthwhile to consider a hidden foci of actinomycosis.
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