The correlation of viral growth capability (n = 156) with the viral load in nasopharyngeal swabs (n = 76) was assessed. Epidemic influenza A/H1N1, A/H3N2, and B viruses showed a wide range of growth capability (104-1011 copies/mL) in Madin-Darby canine kidney cells. The growth was correlated with the nasopharyngeal viral load (r = 0.53). Six selected strains showed growth-dependent cell death (r = 0.96) in a growth kinetics assay. Epidemic influenza viruses exhibit a wide range of growth capability. Growth capability should be considered one of the key factors in disease prognosis.