In 1985 we reported that eosinophilia was found in many children with Mycoplasma pneumonia. We measured the serum concentrations of eosinophil catonic protein (ECP) produced by eosinophils in 25 children withMycoplasmapneumonia, 25 with asthma and 11 with no disease (normal controls). The mean concentrations (×SD) of serum ECP in children withMycoplasmapneumonia, with asthma and the normal controls were 18.7 ≥ 12.6, 23.7 ≥ 12.2 and 6.5 ≥ 1.4 μg/ml, respectively. When compared with those of normal controls, these higher serum concentrations of ECP in children withMycoplasmapneumonia and asthma were statistically significant (P≤ 0.001). The data were directly correlated with the presence of eosinophilia (r= 0.349). There was no relationship between the amount of ECP and the age of children withMycoplasmapneumonia. These results suggest that ECP may work as a factor causing a persistent cough similar to asthma in children withMycoplasmapneumonia.