We present a case of a 55-year-old female patient admitted to the Thoracic Surgery Center for the treatment of a solid left lung mass of unknown aetiology. The patient was transported from another hospital, and the main reason for the patient’s referral to the Thoracic Surgery Center was recurrent haemoptysis and pulmonary haemorrhages. A chest X-ray in the posteroanterior projection showed no changes that could indicate the cause of the patient’s symptoms. In the course of diagnosis, a positron emission tomography combined with computed tomography scan was performed, in which an area of pathological radiotracer uptake was detected in the X segment of the left lung, with transverse dimensions of 6 × 6.5 cm, suggesting left lung cancer. Lower left lung lobectomy was performed, along with mediastinal lymphadenectomy. Final diagnosis was reached by histopathological examination of the resected tissues – the image corresponded to pulmonary actinomycosis.