Background: Low cerebrospinal fluid (CSF) amyloid-β1-42 concentration and high total-tau/Aβ1-42 ratio have been recommended to support the diagnosis of prodromal Alzheimer's disease (AD) in patients with amnestic mild cognitive impairment (aMCI) and also to select patients for clinical trials (Shaw et al, Ann Neurol 2009;65:403-13; Dubois et al, Lancet Neurol 2007;6:734-46). Methods: We tested this recommendation with clinical trials simulations using patients from the Alzheimer Disease Neuroimaging Initiative who fulfilled the following entry criteria: (1) aMCI, (2) aMCI with CSF Aβ1-42 ≤192 mg/mL, (3) and aMCI with total-tau/Aβ1-42 >0.39. For each criterion, we randomly resampled the database obtaining samples for 1000 trials for each trial scenario, planning for 1 or 2 year trials with samples from 50 to 400 patients per treatment or placebo group, with up to 40% dropouts, outcomes after using the AD assessment scale-cognitive subscale and clinical dementia rating scale with effect sizes ranging from 0.15 to 0.75, and calculated statistical power. Findings: Approximately 70% to 74% of aMCI patients with CSF measures met biomarker criteria. The addition of the low Aβ1-42 or high tau/Aβ1-42 requirement resulted in minimal or no increase in the power of the trials compared with enrolling aMCI without requiring the biomarker criteria. Slightly larger mean differences between the placebo and treatment groups fulfilling biomarker criteria were offset by increased outcome variability within the groups. Interpretations: Although patients with aMCI or patients with prodromal AD meeting CSF biomarkers criteria were slightly more cognitively impaired and showed greater decline than patients with aMCI diagnosed without considering the biomarkers, the requirement of biomarker-positive patients would most likely not result in more efficient clinical trials, and trials would take longer because fewer patients would be available. A CSF Aβ1-42 marker, however, could be useful as an explanatory variable or covariate when warranted by the action of a drug. © 2010 The Alzheimers Association. All rights reserved.