An on line clearance monitor automated for measurements of conductivity dialysance (Dcn) was used to measure blood access flow rate (cnQac) during hemodialysis. From mathematical analysis of transport, it was shown that cnQacc = [(Dcn*Decn)/(Dcn - Decn)] [1/bwf], where Dcn is measured with standard cocurrent flow of dialyzer blood (Qb) and Qac; Decn is measured with countercurrent Qb/Qac; and bwf is fractional blood water content. An identical equation was derived to measure Qac from urea dialysance (uQac, Du, Deu). In vitro studies showed excellent correlation between volumetric measurement of Qac (vQac) and cnQac, r = 0.98, n = 29, and between uQac and cnQac, r = 0.97, n = 28. In vivo studies showed comparable agreement between uQac and cnQacc, r = 0.97, n = 14. In two of the patients studied, there was unsuspected severe midgraft stenosis (no recirculation in cocurrent flow) with a Qac of 89 and 202 ml/min disclosed by both cnQac and uQac measurements. Mathematical analysis also showed that when Qb is greater than Qac and there is recirculation in cocurrent flow, the above equations always return the value Qac = Qb. An equation was derived to calculate cnQac without reversal of blood lines in this case, using Dcn calculated from the dialyzer transport coefficient and flow rates.