Please cite this paper as: Mak et al. (2012) The impact of pandemic influenza A (H1N1) 2009 on the circulation of respiratory viruses 2009–2011. Influenza and Other Respiratory Viruses 6(3), e6–e10.Surveillance of respiratory viruses has been conducted for many years at the public health laboratory in Hong Kong. With the occurrence of pandemic influenza A (H1N1) 2009, we observed a change in the seasonality of influenza activity with a seemingly corresponding change in the activity of respiratory syncytial virus, parainfluenza virus, and adenovirus during 2009–2011. This phenomenon could most likely be explained by virus interference.