regions and age groups. The 2-week winter school vacation period in the second half of July 2009 was associated with changes in the age distribution of A/H1N1 cases, and earlier pandemic onset was correlated with large population size in the 24 departments. By contrast there was no association between pandemic dynamics and absolute humidity. Our results suggest that pandemic virus activity began in Lima, consistent with the first case of pandemic A/H1N1 being detected in a Peruvian citizen returning to Lima from New York City on May 9th, 2009. We report a first wave of pandemic A/H1N1 transmission mostly focused on the Lima metropolitan area and peaking in late June, followed by a second wave affecting simultaneously coastal, mountain, and jungle regions of Peru and peaking in mid-July. It is likely that the movement patterns of infected tourists and traders disseminated the infection from the Lima metropolitan area to the rest of the country. A third increase in pandemic A/H1N1 case incidence was reported in the Jungle region in mid to late September 2009 and could be explained by lower population density and reduced connectivity in this region.