Four cases of pulmonary actinomycosis are reported. Two cases (1978, 1979) were diagnosed on microscopic examination of lung parenchyma excised on suspicion of bronchogenic carcinoma. The third case was diagnosed retrospectively by reexamining the slides of a patient operated on in 1965, who presented with a similar history and chest X-ray. The fourth was diagnosed at autopsy in a patient who committed suicide. Actinomycosis is a rare disease which affects the lung in 15--20% of cases. Despite its name, actinomycosis is not due to a fungus but to an anaerobic gram-positive bacillus. The germ is a saprophyte of the human digestive tract and is very sensitive to penicillins. It may become pathogenic in a compromised host and usually produces cervico-facial lesions which develop by continuity from mouth mucosa. Abdominal or, more rarely, pulmonary lesions are due to ingestion or inhalation of infected material. Chest X-ray appearance of pulmonary actinomycosis mimics that of bronchogenic carcinoma. Actinomyces culture is difficult, and diagnosis is often based on microscopic examination of the surgical specimen only. Antibiotic therapy is a mandatory complement to surgical treatment.