To examine their possible predictive value for the development of asthma, the serum concentration of eosinophil cationic protein (ECP) and the total eosinophil count were measured at admission in 25 children aged 1-17 months hospitalized for their first episode of bronchiolitis. After an average of three years the parents of 23 index patients answered a questionnaire to determine development of asthma. Eight children were defined as having asthma at follow-up based on at least three episodes of wheezing. The remaining 15 children had experienced only one or two episodes of wheezing, and all of these children had been wheeze free for the last year. The serum concentrations of ECP were similar in children who subsequently developed asthma (8.0 μg/l; 3.6 to 14.2 (median; quartiles)) and in those who did not (12 μg/l; 4.5 to 16.8). Moreover, the total eosinophil counts were similar in asthmatic (0.10 × 109/l; 0.04 to 0.20) and non-asthmatic patients (0.09 × 109/l; 0.02 to 0.13). In conclusion, our study suggest that neither the serum concentration of ECP nor the total eosinophil count can predict the development of asthma when measured in children admitted for their first episode of bronchiolitis, but larger studies need to be carried out to confirm these results.