OBJECTIVES: In 2009 a novel variant of influenza H1N1 (nH1N1) spread rapidly, causing the first recognised worldwide influenza pandemic since 1968. In measuring the spread of pandemic influenza, the basic reproduction number, R0, is a measure of the number of secondary cases resulting from a single primary case in a naïve population. The purpose of our study was to review published estimates of R0 that relate to the 2009 pandemic providing insight into the relationship between reported R0 estimates and different settings and estimation methods. METHODS: A structured search of published studies in EMBASE.com was undertaken. Studies reporting R0 based on primary data were included. Of those included, we extracted point estimates and measures of variability for R0, in addition to the country, setting, duration and time period for data sources, and the method of R0 estimation. Trends in estimated R0 values by study characteristic were investigated. RESULTS: A total of 184 studies were identified, and 23 studies were included in the review. The reported estimates for R0 ranged from 1.00 to 3.30, with 75% of studies reporting an R0 between 1.22 and 2.3. Studies which used data captured earlier in the pandemic generally estimated higher values than studies which used data captured later in the pandemic. Estimates of R0 were lower in studies that utilised longer term data sets compared to shorter data sets. Bayesian methods estimated lower R0 values than estimations based upon intrinsic growth rates. There was no significant trend in estimates of R0 by country. CONCLUSIONS: The basic reproduction number of nH1N1 may have been overestimated by studies based on shorter duration data sets captured earlier in the pandemic. Awareness of data and methodological restrictions and the effect of this on study results could be vital to accurately predict the spread of future pandemics.