Actinomycosis is an old disease, which was first described in the 19th century; it is one of the most misdiagnosed diseases, even among experienced clinicians. Actinomycosis is a chronic, suppurative granulomatous infection, which is usually caused by Actinomyces israelii. The pulmonary form is slowly progressive, showing nonspecific symptoms at the initial presentation. The disease may spread to the thorax and affect other structures in a characteristic way. Because this infection is relatively unknown, there is a low incidence of cases, but the diagnosis is relatively simple if it is included in the differential diagnosis. We present 3 cases to analyze the most important features of the pulmonary involvement in actinomycosis.