The monoaminergic neurotransmission defect seen in dementia of the Alzheimer type (DAT) is linked to a known increased activity of type B cerebral monoamine oxidases (MAO-Bs). The use of drugs that are able to block this abnormal activity could therefore be useful in the treatment of some cognitive deficits that characterize DAT. Twenty patients with a clinical diagnosis of DAT and with a slight-moderate mental deterioration were treated with 10 mg/day of L-deprenyl, a selective MAO-B inhibitor, according to a double-blind crossover design vs. placebo. Initial treatment (drug or placebo) was randomly assigned. The patients' cognitive functions were evaluated at baseline and then after 3 and 6 months of treatment with drug or placebo. The patients crossed over treatment after 3 months, without a washout interval. The results of the study show the higher and statistically significant effects of L-deprenyl on memory and attention that seem to be due to an improved function of the monoaminergic systems involved in the process of neuronal degeneration.