This study investigated how treatment options, symptom severity, prediagnosis levels of physical functioning, comorbidity, gender, and age predicted current physical functioning in geriatric patients with small-cell or non-small-cell lung cancer, 12 weeks after their diagnosis. The study involved 146 patients aged 65 years and over with an incident diagnosis of lung cancer. Analysis of covariance revealed no significant differences in physical functioning according to treatment type, small-cell versus non-small-cell classification or gender. Significant predictors of current physical functioning were symptom severity and prior physical functioning. The characteristics of a high-risk profile for disruption in physical functioning of elderly lung cancer patients 12 weeks after their diagnosis would include preexisting physical impairment and high levels of symptomatology.