We measured femoral intramedullary pressures and applied axial thrust force generated in vitro during reaming with the AO and Zimmer systems. Six pairs of cadaver femora were instrumented with pressure taps midshaft and in the distal diaphysis, a load cell distally to measure force, and a displacement transducer to monitor reamer position. Following initial hand reaming, intramedullary power reaming was conducted utilizing a 9-mm reamer initially, with subsequent increases in steps of 0.5 mm. All femora were maintained at 37 degrees C and albumin was used to maintain a fluid-filled canal. The highest pressures consistently occurred during initial power reaming, with peak pressures ranging from 270 to 1500 mmHg amongst femora with the AO system. No significant differences were found in the peak pressures generated for the two systems (P = 0.10). The pressure measurements at the two locations in the femur were consistently similar, indicating that pressures are continuous throughout this aspect of the femur. The pressures were not correlated with instantaneous applied axial thrust (R2 = 0.191), and this could be attributed chiefly to the additional friction force of cutting. While pressure generation in the medullary canal upon reaming is likely governed by the rate of clearance of canal content, this is a highly variable response produced by characteristics of the femur which are still not fully understood.