Background.Outbreaks of nosocomial influenza virus infections have been described rarely during childhood and even less so in the neonatal period.Methods.We report 30 neonates admitted to 2 neonatal intensive care units with nosocomial influenza A virus infection, which occurred in 2 outbreaks during 1999. Risk factors for infection were evaluated, and control measures were adopted. Virus was detected by indirect immunofluorescence antibody screen. Any infant with nasopharyngeal aspirate positive for influenza A virus was considered infected.Results.Of 95 infants screened 30 were positive for influenza A virus (31.5%). Mean birth weight was 1622 g, and mean gestational age was 31 weeks in the infected group. In the noninfected group mean birth weight was 2594 g and mean gestational age was 36.4 weeks. Low birth weight, short gestational age, twin pregnancy and mechanical ventilation were identified as risk factors for infection. Clinical symptoms were seen in 22, and 8 were asymptomatic. Clinical features were predominantly respiratory and digestive. The outcome was favorable in all cases.Conclusions.Infection by influenza virus has to be considered as a possible cause of nosocomial infection in the neonatal period. Control measures and prevention are important.