The feasibility of obtaining low-cost high-quality online reinjection fluids was fi rst explored almost 30 years ago, but regulatory conservatism delayed adoption of the technique for almost 20 years. Online treatments are now commonplace in Europe. The competitive advantages of this treatment modality compared to standard convective treatments include lower costs, better quality assurance, a lower environmental burden and better clinical outcomes. The very high volumes of re-infusion fluids peculiar to online treatment allow a better removal of beta2-microglobulin, and there are claims that survival and anemia are better improved by online treatments than by standard convective treatments. In contrast, the acetate burden and its attendant potential hazards are relevant in patients under online treatment, given the considerable quantity of dialysis fluid injected. Acetate-free paired hemodiafiltration, a new online technique, may further ameliorate performances and clinical outcomes, and may actually cut the gordian knot of the safety of online treatments owing to the implemented safeguards.