The cognitive effects of high-dose human leukocyte alpha-interferon (IFN-alpha) treatment were evaluated among 15 patients with the newly diagnosed spinal form of amyotrophic lateral sclerosis (ALS). To confirm the earlier findings showing reversible effects on cognitive performance and to exclude confounding effects, a randomized blinded placebo controlled study was conducted. Twelve patients with continuous intravenous IFN-alpha-infusion treatment over five days and 3 placebo control patients were neuropsychologically evaluated. The neuropsychological examination included tests of intelligence, memory, complex mental processing, visuoconstructional skills, writing, and calculation. A clear difference in the performance profiles of the placebo and the IFN-alpha-treated patient groups was detected: The IFN-alpha group showed significant deterioration during treatment in the digit span backwards task, logical verbal memory task, calculation ability, and writing time, while improvement was seen after treatment. Concomitant fever did not explain the findings. In the placebo group an improvement indicating a learning effect in the three consecutive measurements was found. The reversible cognitive deterioration indicates a clear CNS effect during the IFN-alpha treatment.