Asthma is a disease characterized by chronic eosinophilic inflammation of the airways. Serum eosinophil cationic protein (ECP) has been increasingly used as a noninvasive inflammatory marker in asthma. The serum ECP level seems to reflect, although indirectly, the intensity of ongoing eosinophilic inflammation of the airways and respond sensitively to intervention, whereas it is unlikely to be useful for establishing the diagnosis of asthma in an individual patient. Monitoring of serum ECP could be of utility in the long-term follow-up of asthmatic patients. However, further longitudinal studies are required to establish the role of serum ECP measurement in the treatment modulation in asthma.