The primary objective of the study is to determine whether the incidence of atopic asthma after three years old may be suppressed in the children who were born as preterm infants and prophylactically treated with palivizumab for respiratory syncytial (RS) virus infections during the infancy.

The secondary objective is to determine whether the incidence of recurrent wheezing after three years old may be suppressed in the children who were born as preterm infants and prophylactically treated with palivizumab for RS virus infections during the infancy.

Inclusion Criteria:

• Infants born July ~ December 2007 at 33~35 weeks of gestation
• Infants administered at least 3 doses of palivizumab during the first 6 months of life (Palivizumab treated)
• Infants not administered any doses of palivizumab during the first 6 months of life (Palivizumab untreated)

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)

The children who are participating in another on-going study, "Effect of Palivizumab on Later Recurrent Wheezing in Preterm Infants (ClinicalTrials.gov Identifier: NCT01072552)" and whose parents or legal guardian provide informed consent in writing for continuing participation in the study.