With reversed placement of blood lines and with a peripheral arteriovenous access, hemodialysis recirculation (R) consists of a local access component, and a central cardiopulmonary component that must be separated for the calculation of access flow (Qac) using indicator dilution principles. With indicator injections that follow constant infusion principles Qac = (1 - Rx)/(Rx(1 - CPR)) x (Qb,x - UFR), where Qb is the extracorporeal blood flow, where UFR is the ultrafiltration rate, and where the index x indicates reversed placement of blood lines. CPR, the amount of cardiopulmonary recirculation (CPR = Qac/CO) is determined from two recirculation measurements with correct (index n) and with reversed (index x) placement of blood lines CPR = Rn(1 - Rx)/Rx(1 - Rn) x (Qb,x - UFR)/Qb,n. Qac was measured in 11 hemodialysis (HD) patients using a thermodilution device tested in an in vitro set-up based on constant infusion principles. Mean Qac was 1.135 L/min and 1.054 L/min for measurements done early and late in dialysis. The coefficient of variation was +/-7.3% and +/-8.6%, respectively. Repeated measurements of access flow in HD patients showed good reproducibility (Qac.1 = 1.01*Qac.0, r2 = 0.98), with the regression line not different from the line of identity; however, in vivo results remain to be validated by an independent technique.