To document behavioral effects of interferon (IFN) with daily consecutive assessments we developed an easily administered procedure for bedside testing. It included assessment of mental control and affective behavior, and a self-assessment questionnaire. The method was evaluated in 9 lung cancer patients before and during the treatment with alpha-IFN and the results were supplemented with those obtained by a neuropsychological test battery of one of the patients. The latter tests covered memory, visuoperceptual, visuoconstructional, speech, language, intellectual and psychomotor functions, reading, writing, and calculation. The bedside method was capable of revealing the essential effects of IFN on human behavior, i.e. irritability, slowing of behavior, dyscoordination, and motor perseveration. Consequently, the bedside testing method is recommended for frequently repeated clinical assessments of aberrant behavior of patients treated with IFN, while neuropsychological test batteries serve better for testing situations requiring more detailed and quantified data of localization value.