In treating patients with end-stage renal disease, the dialyzer may be used on multiple occasions rather than once. Long-term effects of this practice are unknown. We report 259 and 1,059 successive patients from facilities practicing reuse in Cincinnati and Detroit, followed, respectively, for 535 and 2,209 patient years. The morbidity was relatively low, expressed by the number of hospital admissions (1.63 and 2.19/year) and by days in hospital (14.24 and 22.71/year), respectively. In Cincinnati the unadjusted case fatality rate was 70% of that in the Ohio Valley Renal Disease Network, in Detroit it was 96% of that in the Michigan Renal Network. There were no adverse long-term effects of multiple use of dialyzers.