The efficacy of acetyl-L-carnitine (gamma-trimethyl-β-acetylbutyrobetaine (Alcar) in mild cognitive impairment (MCI) and mild (early) Alzheimer's disease (AD) was investigated with a meta-analysis of double-blind, placebo-controlled prospective, parallel group comparison studies of at least 3 months duration. The duration of the studies was 3, 6 or 12 months and the daily dose varied between studies from 1.5-3.0 g/day. An effect size was calculated to reflect the results of the variety of measures used in the studies grouped into the categories of clinical tests and psychometric tests. The effect sizes from the categories were integrated into an overall summary effect size. The effect size for the Clinical Global Impression of Change (CGI-CH) was calculated separately. Meta-analysis showed a significant advantage for Alcar compared to placebo for the integrated summary effect [ESall scales=0.201, 95% confidence interval (CI)=0.107-0.295] and CGI-CH (ESCGI-CH=0.32, 95% CI=0.18-0.47). The beneficial effects were seen on both the clinical scales and the psychometric tests. The advantage for Alcar was seen by the time of the first assessment at 3 months and increased over time. Alcar was well tolerated in all studies.