The antioxidant N-acetylcysteine (NAC) or placebo was administered in a double-blind fashion to patients who met National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related Disorders Association criteria for probable AD. Testing for efficacy occurred after 3 and 6 months of treatment. Comparison of interval change favored NAC treatment on nearly every outcome measure, although significant differences were obtained only for a subset of cognitive tasks.