Serratia species are gram-negative bacteria, which could be isolated from soil, water, plants, animals and air. They are responsible for a heterogeneous spectrum of diseases, affecting the central nervous system, the urinary tract, the respiratory tract and the bloodstream. Pulmonary involvement is rare and typically occurs in immunocompromised patients; radiological appearances include haemorrhagic bronchopneumonia, even with the development of pulmonary abscesses and cavitated parenchymal lesions, or diffuse alveolar damage. Concerning pulmonary cavities, the differential diagnosis should include metastatic lung nodules, rheumatoid arthritis, Langerhans cell histiocytosis, mycotic infections and septic emboli. The knowledge of these radiological features, in association with clinical history and laboratory findings, is mandatory to make the correct diagnosis, suggesting the right treatment and the adequate follow-up. We described a challenging case of a Serratia marcescens' pulmonary infection, which occurred in a patient with breast cancer: clinical features and main imaging findings have been discussed - in order to help clinicians and radiologists in the management of the disease.