It is claimed that regular practice of Transcendental Meditation (TM) improves cognitive function and increases intelligence. This systematic review assesses the evidence from randomised controlled trials for cumulative effects of TM on cognitive function. Searches were made of electronic databases and the collected papers and official websites of the TM organisation. Only randomised controlled trials with objective outcome measures of the cumulative effects of TM on cognitive function were included. Trials that measured only acute effects of TM, or used only neurophysiological outcome measures were excluded. 107 articles reporting the effects of TM on cognitive function were identified and 10 met the inclusion criteria. Most were excluded because they used no controls or did not randomise subjects between interventions. Of the 10 trials included, 4 reported large positive effects of TM on cognitive function, four were completely negative, and 2 were largely negative in outcome. All 4 positive trials recruited subjects from among people favourably predisposed towards TM, and used passive control procedures. The other 6 trials recruited subjects with no specific interest in TM, and 5 of them used structured control procedures. The association observed between positive outcome, subject selection procedure and control procedure suggests that the large positive effects reported in 4 trials result from an expectation effect. The claim that TM has a specific and cumulative effect on cognitive function is not supported by the evidence from randomised controlled trials.