The kinetics of 51-Cr-labeled eosinophils were studied in 6 patients with the idiopathic hypereosinophilic syndrome. The kinetics of these cells were compared with 51-Cr-labeled neutrophils of 9 normal subjects. The patients studies consistently showed that autologous labeled eosinophils transiently left the circulating cell pool in the first 3 h after infusion, re-entered the circulating pool and then disappeared from the circulation with a mean blood half-life of 44 .+-. 2.0 h. The neutrophils of normal subjects left the circulation progressively with an estimated blood half-life of 12.4 .+-. 2.0 h. The leukocytosis of the hypereosinophilic syndrome may be due to the presence in the blood of an increased number of cells with a relatively long blood half-life.