We investigated the clinical characteristics separating pneumonia as a complication in elderly lung cancer patients into obstructive and non-obstructive pneumonia. Two hundred and five patients with pneumonia as a complication in elderly lung cancer patients were classified into two groups; 64 patients with obstructive pneumonia and 141 patients with non-obstructive pneumonia. Most of the patients in both groups were male. Concerning histological findings, while most of the patients with obstructive pneumonia had squamous cell carcinoma, those with non-obstructive pneumonia had the same proportion of squamous cell carcinoma as all elderly patients with lung cancer. Most of the patients with obstructive pneumonia were in good general condition including their nutritional condition, but the patients with non-obstructive pneumonia were in significantly poor condition. A low percent of microorganisms were isolated from the sputum obtained from the patients with non-obstructive pneumonia, but a high percentage were obtained from those with non-obstructive pneumonia. Frequent involvement of gram-negative bacilli such as Pseudomonas aeruginosa and Klebsiella pneumoniae or Staphylococcus aureus containing MRSA was also found in these patients. Regarding treatment, although carbapenem was used either alone or in combination therapy as the regimen of treatment for pneumonia as a complication in elderly lung cancer patients with both the obstructive and non-obstructive pneumonia patients, the efficacy rate was poor in 50% with obstructive pneumonia and in 26% with non-obstructive pneumonia. The mortality rate was 11% in the patients with obstructive pneumonia, while it was 61% in the patients with non-obstructive pneumonia. The prognosis was significantly poorer in the patients with non-obstructive pneumonia. We concluded that although the prognosis was not so poor for patients with obstructive pneumonia if the appropriate treatment was given, in the patients with non-obstructive pneumonia, the treatment for underlying diseases and the improvement of their general condition, including the determination of causative microorganisms, was important.