The radiographic and clinical features of 50 patients with documented bacterial lung abscess are presented. Neither clinical nor radiographic features permit a specific diagnosis of lung abscess to be made; microbiologic or histopathologic material is needed to establish the diagnosis. A surprising percentage of patients (18%) had radiographically occult lung abscesses that were diagnosed only at the time of surgery or autopsy. Possible causes for and means of avoiding this diagnostic pitfall are presented.