Peripheral and bone marrow eosinophils were determined in a group of patients on chronic hemodialysis and in predialysis uremics. Healthy subjects were taken as controls. Increased number of eosinophils in bone marrow were found in the predialysis uremic group and this finding was even more accentuated in the dialyzed patients. Marrow eosinophilia correlated linearly with serum creatinine levels in the predialysis group. The peripheral eosinophil count was normal in the uremic group and in part of the hemodialysis group. Marked and progressive peripheral eosinophilia was evident in a distinct subgroup of the dialysis patients. Marrow eosinophilia seems to be part of the uremic syndrome. The possible nature of the factor(s) involved in the derangement of eosinophil homeostasis is discussed.