Infants with acute bronchiolitis during the first months of life are at increased risk of developing persistent wheezing and bronchial asthma later in life. The study of eosinophil cationic protein (ECP) suggests that eosinophil-related inflammatory mechanisms may play a role in respiratory syncytial virus (RSV) bronchiolitis. The aim of our study was to verify whether serum ECP (s-ECP) measurements are useful in predicting the development of persistent wheezing in children affected by RSV bronchiolitis during a 5 years follow-up period. Forty-eight infants were enrolled prospectively (mean age: 153.5 days). All had a clinical and radiological diagnosis of acute bronchiolitis and confirmed RSV infection. Peripheral eosinophil counts, levels of s-ECP, and serum IgE concentrations were measured during bronchiolitis. Five years later the children were re-evaluated in regard to their respiratory symptoms (standardized questionnaires) and atopic status (specific IgE levels). We observed significantly higher s-ECP levels (P < 0.001) at enrollment in subjects who developed persistent wheezing compared to subjects who did not show late wheezing. Initial s-ECP values allowed significant and correct prediction of persistent wheezing (P < 0.001). The risk to develop respiratory symptoms was 9.73 higher for infants with s-ECP levels > or = 8 microg/L than for those with s-ECP levels <8 microg/L (P < 0.0001). In conclusion, our study suggests that s-ECP levels in infants with bronchiolitis are useful in predicting the risk to develop wheezing in the subsequent 5 years.