The hypothesis of the present study is that the prophylaxis with palivizumab to prevent the severe RS virus infection during the infancy among preterm infants may reduce the risk of subsequent recurrent wheezing in childhood.

The infants born between July 1st and December 31st in 2007 with the gestational age between 33 and 35 weeks were enrolled into the study at the end of RS virus infection season, April 2008. The infants were unintentionally divided into two groups, either palivizumab treated or untreated group at the enrollment, because the timing for palivizumab prophylaxis were already ended.

The study infants will be followed up until the age of 3 with recording the incidence of either parent reported or physician diagnosed recurrent wheezing.

The difference of the incidence of the recurrent wheezing between the groups will be analyzed with Kaplan-Meier method.

Inclusion Criteria:

• Infants born July ~ December 2007 at 33~35 weeks of gestation

Exclusion Criteria:

• Intrauterine growth retardation (less than -2.5SD)
• Infants with chronic lung disease (CLD) or other respiratory disease
• Infants received mechanical ventilation.
• Infants with chronic heart disease (CHD) or congenital anomaly (such as immunodeficiency).
• Infants received less than 3 doses of palivizumab during the first 6 months of life

Infants with the gestational age between 33 and 35 weeks