Thin-walled cavity with air-fluid level is often considered radiographically benign-looking lesion. We recently encountered a patient with a rare lung cavity. A 58-year-old male presented with intermittent fever, chest pain and hemoptysis. A large thin-walled cavity, with a smooth inner surface and a clear air-fluid level, occupied the left upper lobe on the chest computerized tomography (CT) scan. After intravenous antibiotics was administrated, the air-fluid level in the cavity disappeared, and the cavity was gradually fluid refilled unexpectedly. However, the cavitary lesion kept the previous contour. Then CT-guided percutaneous needle lung biopsy was performed, revealing a squamous cell carcinoma of the lung. Although it is rare, a refilled thin-walled cavity in lung may be malignant, which should be kept in mind by clinician.