We conducted a prospective study of cognitive function in patients with small cell lung cancer (SCLC). A previous study had shown SCLC patients to have deficits in memory, frontal lobe executive functioning, and motor skills before they received prophylactic cranial irradiation (PCI). This study was performed to determine whether these deficits were related to chemoradiation treatment. We evaluated two groups of patients: one newly diagnosed and untreated, the other post-chemoradiation and before PCI. Strict eligibility criteria were used to minimize any preexisting factors that might influence cognitive functioning. Results showed that patients with SCLC have cognitive deficits before receiving chemoradiation, specifically on tests of verbal memory, frontal lobe executive functions, and motor coordination. There was no difference between the treated and untreated patients on any test. Etiologic considerations currently under study include paraneoplastic phenomena.