A retrospective study of 1011 hospitalized patients with pneumonia was undertaken to assess the value of routine convalescent chest radiography for detection of underlying lung cancer. To investigate the mode of clinical onset of pulmonary carcinoma, 232 inpatients with this diagnosis were also studied. The findings may be summarized as follows: 1) 13/1011 pneumonia patients were found to have previously undiagnosed pulmonary carcinoma; 2) many of these carcinomas (8/13) were disclosed by an acute chest X-ray; 3) pulmonary carcinoma was found by convalescent chest X-ray in 2/88 patients not feeling well and in 2/524 patients feeling well at follow-up, and none of these 4 patients benefitted from the carcinoma diagnosis; 4) ESR was of no value in detecting underlying pulmonary carcinoma at follow-up in patients with pneumonia; 5) of the 232 patients with pulmonary carcinoma, 29 (12.5%) presented with an acute respiratory tract infection; 6) most of these latter patients did not recover as expected and their correct diagnosis was made based on a chest X-ray performed because of persistent symptoms. We suggest that patients with radiologically verified pneumonia undergo clinical examination or are interviewed 4-5 weeks after the onset. If signs or symptoms of respiratory disease persist, chest X-ray should be performed. We consider, however, that routine convalescent chest radiography with the aim of detecting any underlying pulmonary tumour could be omitted if the patient has completely recovered 1 month after the acute onset of illness.