We analyzed serum levels of eosinophilic cationic protein (ECP), one of the 4 main eosinophilic proteins; ECP is released from an activated cell and acts as a mediator of inflammation. Serum samples from 139 persons were studied prospectively. Fifty-three individuals from the general population provided the control group. Eighty-six consecutive patients were also studied: 69 with bronchial asthma and 17 with allergic rhinitis and no signs of asthma. The level of severity of disease was established in the asthmatics by the method proposed by Aas (Aas score), based on symptoms and medications received within the last year. We also classified these patients as having mild, moderate or severe asthma according to the latest criteria issued by the International Consensus for Diagnosis and Treatment of Asthma. Atopic status was estimated by skin Prick tests. ECP levels in the control group (9.34 +/- 5.76 micrograms/l) were significantly lower (p < 0.001) than those of the total population of patients (17.59 +/- 16.85 micrograms/l). The mean for patients with rhinitis was 14.76 +/- 10.94 micrograms/l, whereas it was 18.29 +/- 18 micrograms/l in the asthmatics; the levels for both groups were statistically different from that of the control group (p < 0.03 and p < 0.001, respectively). Levels by degrees of severity established at the time of revision and by sensitivity to allergens were also significantly different from the level of the control group, although the mean levels were low in the group of severely affected patients who had received treatment with inhaled corticoids.(ABSTRACT TRUNCATED AT 250 WORDS)