In many cases, the diagnosis of eosinophilic myocarditis is suggested by an elevated peripheral blood eosinophil count. However, no detailed studies have been performed on the sequential changes in the initial peripheral blood eosinophil count over the course of the disease. We measured the peripheral blood eosinophil count at the time of presentation in eight patients with eosinophilic myocarditis proven by endomyocardial biopsy and intermittently thereafter. The eosinophil count at the time of onset was <500/mm(3) in four patients, >500/mm(3) but <1,000/mm(3) in three patients, and > or =1,000/mm(3) in one patient. In three of the four patients with an initial eosinophil count of <500/mm(3), an increase to > or =500/mm(3) occurred 7-12 days after the onset. The remaining patient did not develop peripheral eosinophilia. In conclusion, in the early stage of eosinophilic myocarditis, peripheral hypereosinophilia is not present initially in some patients, and may not develop during the course of the illness in a subset of these patients.