In traditional Chinese medicine (TCM), the imbalance of yin and yang is one of the basic pathogeneses of a disease. Preponderance of yang leads to "heat" manifestations including thirst, dryness of the throat, dark scanty urine and constipation. Treatment of asthma in TCM is based on the differentiation of "heat" Zheng according to the manifestations. Some of the patients with allergic asthma also present typical "heat" manifestations. To investigate the essence of "heat" manifestation in asthma, we measured the serum level of eosinophil cationic protein (ECP) in asthmatic patients. ECP usually represents the activation of eosinophils which are the main effectors in late allergic reactions. Our results demonstrated that asthmatic patients with "heat" manifestations had higher serum ECP levels, compared to those without "heat" manifestations (34.3 +/- 4 microg/l versus 15.3 +/- 3 microg/l). However, total immunoglobulin B (IgE), and the eosinophil count in peripheral blood did not show any difference between the "heat" and "non-heat" groups. Therefore, we conclude that ECP in asthmatic patients plays an important role in the development of "heat" manifestations as diagnosed by TCM.