119 patients were enrolled in a double-blind randomized parallel study versus placebo carried out to assess both the efficacy and tolerability of L-deprenyl (10 mg/day) for treatment of patients with organic mental disorders of the Alzheimer type (DAT). The treatments were given for 3 months, starting after a run-in period of 15 days to evaluate efficacy. A complete neuropsychological battery was administered monthly after the start of treatment whereas tolerability was assessed by checking, recording and classifying all the unfavorable experiences occurring. According to the results, L-deprenyl would seem to be a useful and reliable tool for the treatment of DAT patients in an attempt to improve their cognitive functions and reduce behavioral alterations, without frequent or severe side effects.