In this randomized double-blind, placebo controlled, multicentre study on 108 elderly patients with mild to moderate mental deterioration of vascular origin, idebenone - a benzoquinone derivative with a hydroxyalkyl side chain - proved to be therapeutically effective in the treatment of multi-infarct dementia. The oral administration of idebenone 45 mg/day b.i.d. for 120 days significantly improved the scores of the following test in comparison with placebo: Mini Mental State, Randt Memory Test, Gottfries Rating Scale, Token Test, Toulouse Piéron Test, indicating improvements in memory attention and cognitivity. The drug was well tolerated and effective in patients with multi-infarct dementia. No changes in laboratory parameters were observed either before or after treatment.