Absolute humidity was shown to affect influenza survival and transmission in previous studies [24] , and was linked to the timing of onset of seasonal and pandemic influenza outbreaks in the US [11, 12] . Here we explored the relationship between daily variation in absolute humidity and the temporal profile of the pandemic in the 134 provinces of Peru. For this purpose, we compared daily variation in number of new A/H1N1 influenza cases and average specific humidity in Peru weighted by the total number of A/H1N1 cases reported in each province from May 1 to December 31, 2009. Specific humidity (g/kg) is a proxy for absolute humidity and was calculated from daily averages of temperature, relative humidity, and surface pressure obtained from the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) global reanalysis [25] .