Inhaled nonisotonic solutions (e.g., ultrasonically nebulized distilled water, UNDW) are sensitive bronchoconstrictive agents in asthmatics, their suggested mechanism of action being the release of mediators from some inflammatory cells. Studies assessing the relationship between degree of bronchial hyperresponsiveness to UNDW and plasma levels of eosinophilic markers are not available to date. Fourteen asymptomatic, nonsmoking, naive asthmatics (8 males, aged 18 to 45; mean basal FEV1 = 93.8% predicted +/- 1.7 SE), monosensitized to Dermatophagoides pteronyssinus, and twelve normal subjects (9 males, aged 19 to 40, mean basal FEV1 = 93.3% predicted +/- 1.4 SE) entered the study after informed consent. Each subject performed the UNDW challenge while the transcutaneous pO2 (PtcO2) was monitored. Serum eosinophil cationic protein (ECP) was measured together with blood eosinophils 60 min before UNDW. The bronchial response was expressed as FEV1 and PtcO2% drop from baseline, as assessed 10 min following UNDW. Mean blood eosinophils were 3.4% total leukocytes +/- 0.3 SE in normal subjects and 7.9% total leukocytes +/- 0.4 SE in asthmatics (p < 0.001). Mean serum ECP was 4.6 micrograms/l +/- 0.6 SE in normal subjects and 22.4 micrograms/l +/- 1.3 SE in asthmatics (p < 0.001). Mean FEV1 drop was 2.1% +/- 1.1 SE in normal subjects and 24.2% +/- 1.1 SE in asthmatics; the corresponding mean PtcO2% drop was 3.6 +/- 0.7 SE in normal subjects and 28.6 +/- 1.4 SE in asthmatics. Serum ECP was found to be related to blood eosinophils (r = 0.7, p = 0.003); despite the eosinophils, serum ECP proved to be related to a drop in both FEV1 and PtcO2, atr = 0.6 (p = 0.024) and r = 0.7 (p = 0.006), respectively. In conclusion, serum ECP, but not eosinophils per se, can differentiate normal subjects from naive asthmatics hyperresponsive to UNDW. Serum ECP was also proven to be directly related to both the UNDW-induced bronchoconstriction and hypoxemia, thus confirming that proximal, but also the more peripheral airways, are actively involved. Bronchial hyperresponsiveness to UNDW can then be considered an effective reflection of the existence of underlying inflammation of the airways.