Neuropsychological analysis of 47 cancer patients with widely metastatic disease and significant previous treatment indicated that 34% had cognitive deficits. Previous treatment with biologic response modifiers was associated with a 53% frequency of cognitive abnormalities, whereas only 18% of patients who were never treated with biologics had such impairments. The possibility that neurobehavioral abnormalities will result from metastatic disease and treatment regimens is an important factor in determining the risks and benefits of therapy.