We report a case of spontaneous interlobar pneumothorax, an uncommon condition. The diagnosis was suspected from the frontal chest X-ray, which showed an incompletely circumscribed air-containing space, with a fluid level. The diagnosis was confirmed by the lateral projections of standard X-ray and tomography, and by thoracic computed tomography (CT). Such air-fluid levels should not be confused with pneumatocele, cystic or cavitary diseases.