Nine cases of thoracic actinomycosis treated in the period 1966-1987 are reported. Three of the patients presented with a clinical picture of empyema and six were admitted for suspected primary pulmonary neoplasm. No case was correctly diagnosed preoperatively. The final diagnosis was based on direct microscopy (2) or culture (1) of drained pus in the empyema cases and on histologic examination of resected tissue in the others. In one case the correct diagnosis was missed when a palpable tumour mass was not extirpated at exploratory thoracotomy, and was made only when clinical deterioration necessitated a second operation. Good clinical results can be expected from surgical eradication of an intrathoracic focus of actinomycosis in conjunction with appropriate antibiotic therapy.