The relationship between the blood flow and inflow and outflow pressures was determined in PermCath, dual lumen catheters during regular hemodialyses in vivo in eight patients with average hematocrit of 38%. From the luer lock connector the catheters had an average length of 32 cm to the outflow tip and 30 cm to the inflow tip. The catheters had an internal diameter of 0.2 cm and were straight before implantation. Dialyses were performed on Fresenius 2008 D or E machines with ReadySet blood lines with an 8 mm ID pump segment and a noncollapsible arterial chamber. Pressures and blood flows were measured at pump speeds from 50 to 500 ml/min in increments of 50 ml/min with lines in normal configuration. Blood flow was measured continuously using ultrasound. The correlations between pressures and flows are not linear. The best correlations are according to the Stirling model of exponential growth category equation. Inflow pressure = -9.07-0.4865*(exp(0.0020*blood flow)-1)/0.0020 Outflow pressure = -28.14+0.5002*(exp(0.0015*blood flow)-1/0.0015 Based on these results and Poiseuille's equation a table was developed for the optimal relationship between catheter length and diameter to achieve standardized (average, low and high) blood flows regardless of the lengths of the catheters. The diameter/length relationships are based on theoretical considerations. Because resistances depend on the material and shape of the tubing, the actual measurements of flow/pressure relationships should be done once tubings of different diameters are manufactured, and final catheter design has to be based on these measurements.