Integrated positron emission tomography combined with plain computed tomography is commonly employed as the diagnostic tool for patients presenting with lung opacities. This technology is non-invasive and does not put the patient at risk of contrast reaction. We report a case of a man who presented with fever, septicaemia, and a left apical lung opacity on chest radiography. His positron emission tomography and plain computed tomography scans showed increased uptake by the left apical lung opacity together with a huge anterior mediastinal mass, suggestive of lung cancer with mediastinal lymph node involvement, and a right upper lobe shadow. After an initial futile bronchoscopy, an endobronchial ultrasound-guided transbronchial needle aspiration of the mediastinal node was planned but a contrast computed tomographic scan of the thorax revealed no significantly enlarged mediastinal lymph nodes. The differential diagnoses of these findings, together with the limitations of positron emission tomography and plain computed tomography, are discussed.