Background: Millennium Development Goal 4 calls for a reduction in the under-5 mortality rate (U5MR) by two-thirds between 1990 and 2015. Due to issues with data quality and quantity, there is uncertainty about how much progress has been made in many countries, and conflicting estimates of the overall levels and rate of decline have been reported. Consequently, there is a clear and urgent need to validate estimates, but consensus guidelines for model validation are lacking. Methods: We propose a validation method, based on the question: If current U5MR estimation methodology had been used in the past, using all available data available at that time, how accurate would the U5MR estimates and uncertainty intervals have been?Findings: Validation of the estimates and uncertainty intervals reported in 2010 by the Institute for Health Metrics and Evaluation showed that estimates of both the past absolute levels and the rate of decline in under-5 mortality between 1990 and 2000 would have changed significantly between 2004 and 2010 for many countries, with the inclusion of additional data. For high-mortality countries in particular, over 40% of the updated estimates for the U5MR in 2000 fell outside previously constructed 95% uncertainty intervals, and the absolute change in the estimated rate of decline was more than 1% for about three out of five countries.Interpretation: In light of evaluating Millennium Development Goal 4 in 2015, we hope that extensive model validation exercises will become standard practice for validating the accuracy of under-5 estimates and uncertainty intervals. Ultimately the goal of including appropriate validation methods is to improve the reliability of both estimates and uncertainty intervals, which will allow for a more fully informed assessment of both the absolute levels of, and rates of change in, the under-5 mortality rate.