Four influenza A challenge studies were performed over a period of three years using the same dose of one virus pool. The first three studies were conducted two influenza seasons apart from the last study. In all four studies only subjects with screening hemagglutination inhibition (HAI) antibody titers less than or equal to 1:8 in sera were accepted as study subjects. The rate of infection after influenza challenge was 96% (25 of 26) in the first three studies, and only 73% (8 of 11) in the last study (P less than 0.04). The rate of influenza illness in the first three studies was 62% (16 of 26), and only 9% (1 of 11) in the last study (all subjects: P = 0.003; infected subjects: P = 0.01). Even though screening HAI titers were comparable between groups, prechallenge serum influenza-specific IgG titers correlated inversely with respiratory symptoms (P = 0.04). Prechallenge nasal wash influenza-specific IgA titers were lower in subjects who developed influenza illness (P = 0.03). Prechallenge influenza-specific nasal wash ELISA-IgA titers and serum ELISA-IgG titers predicted influenza severity in patients prescreened by HAI during influenza challenge studies.