The cytotoxic proteins released by activated eosinophils should play a role in the development of Churg-Strauss syndrome (CSS). Eighteen patients (15 males and 3 females, age 41 +/- 13.3 years) with CSS according to the American College of Rheumatology criteria were included in the study. Thirteen serum samples from 11 patients were obtained at the time of disease flare, and the sera from 6 of them were also obtained at the time of clinical remission. Sera from seven other patients were obtained in clinical remission. Anti-neutrophil cytoplasm antibodies were detected in four (22.2%) patients. Fifteen healthy individuals were used as controls. Mean eosinophil count differed significantly between CSS patients with active disease and patients in clinical remission (3,407/mm(3) vs. 258/mm(3); P < 0.01), between CSS patients with active disease and healthy individuals (3,407/mm(3) vs. 211/mm(3); P < 0.01). Mean serum ECP levels differed significantly between patients with active or inactive disease (219 microg/L vs. 56.8 microg/L; P < 0.0001), between patients with active disease and healthy individuals (219 microg/L vs. 26.2 microg/L; P < 0.0001), but not between patients with inactive disease and healthy individuals (ns). Peripheral blood eosinophils count correlated with serum ECP during CSS flares disease (R = 0.6264; P < 0.05) and during periods of remission (R = 0.4798; P < 0.05). Our results support that ECP might be used as a disease activity marker in CSS.