The largest study on cyclophosphamide pharmacokinetics in dialysis patients comprises of 6 subjects. In the 2 decades since these data were obtained, dialyser membranes, treatment intensities, and treatment duration have changed considerably making new pharmacokinetic studies desirable. We aimed to readdress the pharmacokinetics of cyclophosphamide in a 74-year-old critically ill male suffering from ANCA-associated vasculitis. Due to an acute-on-chronic kidney injury, he underwent intermittent (IHD) and prolonged intermittent kidney replacement therapy (PIKRT). IHD was started 7 h after end of a cyclophosphamide infusion with a blood/dialysate flow of 300 mL/min for 255 min, followed by PIKRT with a blood/dialysate flow of 140 mL/min for 540 min, both using a 1.3 m2 polysulphone high-flux dialyser (F60S, Fresenius Medical Care). Peak concentration of cyclophosphamide was 20.2 mg/L. Using IHD and PIKRT serum concentration of cyclophosphamide decreased to 1.2 mg/L after IHD and to <0.1 mg/L after PIKRT with dialyser-clearances of 153.0 mL/min and 84.9 mL/min, respectively. Total recovery of cyclophosphamide, calculated from the collected dialysate, was 57.5 mg (7.7% of administered dose) for IHD and was 8.3 mg (1.1% of administered dose) for PIKRT. By using IHD with a high-flux dialyser cyclophosphamide could be eliminated. Remaining cyclophosphamide should be eliminated by PIKRT. Hence, even in the absence of renal function a dose >50% of the recommended for patient with normal renal function may be applied, as complete elimination of the parent drug by modern dialysis is feasible.