['There is evidence to suggest that component randomized controlled trials (RCTs) within systematic reviews may be biased. It is important that these reviews are identified to prevent erroneous conclusions influencing health care policies and decisions.', 'To assess the likelihood of bias in trials in 12 meta-analyses.', 'A review of 12 systematic reviews.', 'Twelve recently published systematic reviews with 503 component randomized trials, published in the British Medical Journal, The Lancet, Journal of the American Medical Association, and The Annals of Internal Medicine before May\xa02012.', 'Systematic reviews were eligible for inclusion if they included only RCTs. We obtained the full text for the component RCTs of the 12 systematic reviews (in English only). We extracted summary data on age, number of participants in each treatment group, and the method of allocation concealment for each RCT.', 'Five of the 12 meta-analyses exhibited heterogeneity in age differences (I(2)\xa0>\xa00.30), when there should have been none. In two meta-analyses, the age of the intervention group was significantly greater than that of the control group. Inadequate allocation concealment was a statistically significant predictor of heterogeneity in one trial as observed by a metaregression.'] Does a methodological review of recent meta-analyses have found significant heterogeneity in age between randomized groups?