Aim: To evaluate whether the presence of immunoglobulin G (IgG) antibodies ag ainst respiratory syncytial virus (RSV) in early childhood is associated with la ter asthma, and to evaluate a new diagnostic test for RSV, reverse- transcripti on polymerase chain reaction (RT- PCR), comparing it to the antigen and antibod y assays initially used in RSV diagnostics in the present cohort. Methods: At th e start of the study in 1992- 1993, RSV was studied by antigen detection (using time- resolved fluoroimmunoassay) and complement- fixing antibody assay. Adva nces in methodology allowed us to supplement RSV studies by RTPCR in frozen naso pharyngeal aspirates obtained on admission, and by specific IgG antibodies (usin g enzyme immunoassay) in frozen serum samples obtained during the follow- up. R esults: On admission, 29 of the 100 children hospitalized for wheezing at < 2 y of age were RSV positive. When compared with conventional methods, the sensitivi ty of RT- PCR was 83% (100% w.r.t. antigen detection) and its specificity was 92% in diagnosing RSV infection. RSV- s pecific IgG antibody concentrations rose with age, but were not predictive of as thma at any age. In the present cohort, wheezing without RSV was particularly as sociated with increased risk for later childhood asthma. Conclusion: Hospitaliza tion for wheezing in infancy is associated with increased risk for later childho od asthma, particularly in children without RSV infection on admission, although children with RSV have also slightly increased risk for later asthma. However, mere serological evidence of RSV infection is not associated with the developmen t of asthma. In addition to RSV, more attention should be paid to less virulent agents in order to find those wheezing infants who are at particular risk of lat er childhood asthma. Aim: To evaluate whether the presence of immunoglobulin G (IgG) antibodies ag ainst respiratory syncytial virus (RSV) in early childhood is associated with la asthma, and to evaluate a new diagnostic test for RSV, reverse-transcripti on polymerase chain reaction RT-PCR), comparing it to the antigen and antibod y assays initially used in RSV diagnostics in the present cohort. Methods: At first start of the study in 1992- 1993, RSV was studied by antigen detection (using time- resolved fluoroimmunoassay ) and complement-fixing antibody assay. Adva nces in methodology allowed us to supplement RSV studies by RTPCR in frozen naso pharyngeal aspirates on admission, and by specific IgG antibodies (usin g enzyme immunoassay) in frozen serum samples obtained during the follow-up Résults: On admission, 29 of the 100 children hospitalized for wheezing at <2 y of age were RSV positive. When compared with conventional methods, the sensitivi ty of RT- PCR was 83% (100% wrt anti gen detection) and its specificity was 92% in diagnosed RSV infection. RSV-s pecific IgG antibody concentrations rose with age, but not not predictive of as thma at any age. In the present cohort, wheezing without RSV was particularly as sociated with increased risk for later childhood asthma. Conclusion: Hospitaliza tion for wheezing in infancy is associated with increased risk for later childhood asthma, particularly in children without RSV infection on admission, although children with RSV have also slightly increased risk for later asthma. However, mere In addition to RSV, more attention should be paid to less virulent agents in order to find those wheezing infants who are at particular risk of lat er childhood asthma.