OBJECTIVES: The reliability of the results of a randomized trial depends on the extent to which potential sources of bias have been avoided. We tested the EUnetHTA Internal Validity guideline so that to harmonize our risk of bias assessments with the European standard and finally to improve the reliability of relative efficacy and cost effectiveness assessments for decision makers in Hungary. METHODS: We translated the risk of bias standardized assessment questions of the EUnetHTA Internal Validity (of randomized controlled trials) into Hungarian. We first chose ten studies for internal validity testing from the ones that were submitted for reimbursement at the beginning of 2012, and their results were used for health economy assessment. RESULTS: We found adequate randomization sequence generation in seven studies and we marked it unclear in three trials (e.g.: lack of information, age related sequence generation). The allocation concealment was labeled suitable in six studies (e.g.: IVRS, IWRS) and unclear in four trials. All studies could be classified according the the type of blinding. We found selective reporting in one trial where the non-inferiority results in the per-protocol population were not published. We rated the  risk of bias low for eight trials and high for two trials due to unclear sequence generation and publication bias. We also evaluated 77 endpoints and we labelled 22 endpoints with high risk of bias. The most common reasons for high risk ratings were the not appropriately implemented ITT principle and selective reporting. CONCLUSIONS: The EUnetHTA guideline gives an opportunity to estimate the risk of bias of randomized controlled trials in a structured and harmonized way without leaving out any important considerations. The results of the internal validity evaluation can lead the focus of interest to those endpoints where the sensitivity analysis is requisite in the health economic models.