Neuropsychologic tests were performed in subjects with Down syndrome in order to assess the effect of a 90-day treatment with L-acetyl-carnitine (LAC). Findings were evaluated statistically (Wilcoxon test) and compared to three further groups of subjects: untreated Down syndrome, mental deficiency due to other cases treated and not treated with LAC (Mann-Whitney U-test). Treated Down syndrome patients showed statistically significant improvements of visual memory and attention both in absolute terms and in comparison with the other groups. No improvement was found in mentally deficient non-Down subjects, so that the favourable effect of LAC appears to be specific for Down patients. In view of the analogies of the pathology and neurochemistry between Down syndrome and Alzheimer degenerative deficiency (deficit of cholinergic transmission) it is suggested that the action of LAC in these pathologies is related to its direct and indirect cholinomimetic effect.