Influenza A virus is an important cause of acute respiratory infections (ARI). Clinical manifestations of ARI vary from mild or moderate to life-threatening conditions requiring intensive care. Given the segmented genome, a large natural reservoir of other influenza virus subtypes, and antibody selection pressure in the population, the virus is variable and genetically unstable. The phylogenetic analysis and genotyping of A/H3N2 influenza viruses isolated from patients hospitalised with influenza-like illness symptoms in the Na Bulovce Hospital in the season 2011/2012 support the assumption that the pathogenicity is a polygenic trait modifiable by the host health status and seems not to be unambiguously associated with any specific mutations.