Eosinophilia (E) has been noted in hemodialysis (HD) patients, but its etiology is not clear. In an effort to clarify this phenomenon, we prospectively studied patients initiating dialysis in our outpatient HD and peritoneal dialysis programs. Rate of E was greatest for a small group of four continuous cycling peritoneal dialysis patients (75%), less for 63 HD patients (41%), and least for 66 continuous ambulatory peritoneal dialysis (CAPD) patients (21%, P less than .05, HD v CAPD). Increasing E rates among the groups paralleled increased frequency of tubing changes. There were no differences in etiology of renal disease, medications, types of dialyzers, types of access, or transfusion frequency that could account for the E. IgE levels did not correlate with E. The data suggest that the dialysis procedure or the tubing changes may be causing the E, but the possibility that uremia, itself, is important in the pathogenesis of dialysis E is also discussed.