By using the microsuction technique, a quantitative determination of eosinophil cationic protein (ECP) in nasal secretions was performed in 18 atopic patients and 10 healthy volunteers. The percentage of the eosinophils was counted by simple microscopic examination. In the control group, results showed that the percentage of the eosinophils was almost 0 (range: 0-0.7%), and the median baseline ECP concentration was measured at 105 ng/g (range: 2-281 ng/g). No significant changes in either eosinophils or ECP were observed in this group. In the patient group, there existed a significantly higher baseline percentage of eosinophils (median: 2.7%, range: 0-45%) than in the control group but not for baseline ECP concentration (median: 160 ng/g, range: 6-1475 ng/g). One hour after nasal allergen challenge (NAC), a significant (P < 0.01) increase was seen in the percentage of eosinophils of the nasal secretions and this was remained for 10 hours (median: 5.5-13.3%). However, the difference that occurred was that ECP reached its highest concentration (median: 401 ng/g, range: 32-2298 ng/g) only 24 hours after NAC, while the percentage of the eosinophils decreased. Our findings again support a role of migration and activation of eosinophils in the pathogenesis of nasal allergic reaction, especially during the late phase.