BACKGROUND: In the Netherlands in 1997, 43% of patients with newly diagnosed lung cancer were over 70. Large age-specific differences in treatment exist. We examined whether age, comorbidity, performance status and pulmonary function influenced treatment. PATIENTS AND METHods: Data on patients with newly diagnosed non-small cell lung cancer (N = 803) were obtained: comorbidity, performance status, pulmonary function (FEV1) and initial treatment. Age-specific differences in treatment according to the guidelines were examined. Odds ratios were calculated by means of logistic regression analyses. RESULTS: 82% with stage I or II disease received treatment according to the guidelines; this applied to 48% with stage IIIA disease and to 54% with stage IIIB disease. For all stages, this proportion decreased with increasing age. In stage IV disease, 36% did not receive any treatment; this applied to 52% of the elderly patients (75+ years). Multivariate analyses showed associations between comorbidity and treatment choice, but none with performance status. Age of 75+ years appeared to be the most important factor for not receiving treatment according to guidelines. CONCLUSION: A substantial proportion of elderly patients with non-small cell lung cancer did not receive standard treatment. Performance status and comorbidity seldom formed the underlying reason. Calendar rather than biological age seemed to play the most important role in choice of treatment for patients with non-small cell lung cancer.