Meta-analysis is a valuable method of aggregating data sets of different trials which are inadequate or unconvincing on their own. There are strict rules on the retrieval and selection of data which are crucial in terms of the validity of a meta-analysis. Many so-called meta-analyses are flawed because of lack of a protocol or non adherence to it. Also underreporting of clinical trials is haunting the validity of meta-analysis. This hidden information is not a random process but is affecting mainly trials with negative results. To avoid this publication bias all clinical trials should enter an international online database.