Delayed central venous perforation is an uncommon but serious complication of central venous catheter insertion. An increase in catheter stiffness may have been responsible for our association of venous perforation with use of a guidewire insertion technique. A bench model was used to investigate the stiffness characteristics of thirty-four different types of catheters. The initial stiffness is poorly described by material or catheter gauge. A large range of values is seen between apparently similar catheters - the 16 gauge polyethylene catheter associated with two perforations at our institution had an initial stiffness value 7.5 Nm2 x 10-5 at 37°C in comparison with our previous standard - the 16 gauge Deseret Intracatch with an initial stiffness of 2 Nm2 x 10-5. Multilumen catheters had a similar range of stiffness to single lumen cetheters, while paediatric catheters in general were less stiff. Dialysis catheters were up to five times as stiff as the stiffest central venous catheter. Stiffness decayed at a rate and to an extent which differed from catheter to catheter. Absorption of water by the catheter appears to be one factor involved in stress relaxation.