Thoracic actinomycosis is an uncommon disease, which may mimic malignancy, lymphoma or tuberculosis of the chest. In the past three years, four cases of thoracic actinomycosis have been found in children at our hospital. Their computed tomography (CT) findings included pulmonary infiltrates, a chest wall mass, pleural and pericardial effusion, mediastinum involvement and rib changes. Although the final diagnosis of Actinomyces infection depends on a bacterial culture and pathology, CT can play an important role in establishing the diagnosis and evaluating the extent of the disease.