Neuropsychological tests were used to evaluate possible central nervous system dysfunction in patients treated with chemotherapy. Ninety-five patients with non-small cell lung cancer limited disease were randomized to either radiotherapy (2.8 Gyx15) or combination chemotherapy with cisplatin and etoposide. In order to evaluate cognitive functions three neuropsychological tests were applied: Trail Making, Benton Visual Retention Test and Verbal Learning. Changes in the patients' test scores before and after treatment were compared. The chemotherapy patients showed reduced performance on some of the neuropsychological tests compared to the radiotherapy group. This indicates a treatment related effect on the central nervous system, possibly caused by the combination chemotherapy.