Objective: To examine the cognitive and behavioral effects of L-deprenyl in persons with mild dementia of the Alzheimer type (DAT) over a 15-month period. Design: A 15-month randomized, double-blind, placebo-controlled trial using a parallel-group design. Participants: 39 subjects with mild DAT (CDR 1) selected using NINCDS-ADRDA criteria. Outcome Measures: A battery of neuropsychological tests and dinical rating scales. Results: The placebo and L-deprenyl subjects were similar at baseline on the clinical and neuropsychological tests. The total score on the Brief Psychiatric Rating Scale (BPRS) was significantly less after 15 months in subjects taking L-deprenyl. The only individual item of the BPRS which differed between groups was disorientation. There was no evidence of a L-deprenyl effect on any other clinical or neuropsychological measure after 2, 8, or 15 months. Conclusion: L-deprenyl did appear to have a slight effect on a single measure of psychopathology, the BPRS. It did not have a measurable impact on any other measure of behavior or cognitive function over a 15-month period in this group of subjects with mild DAT and did not appear to slow the progression of the disease.