OBJECTIVE: Research has demonstrated the utility of the Pain Behavior Measurement (PBM) system as a pain index. PBM involves the recording of sighing, rubbing, grimacing, guarding and bracing. A modification of this system has been proposed, focusing on the occurrence of joint flexing, rubbing, unloading the joint, guarding and rigidity, specifically for patients with knee pain. The aim of the present study was to compare the original PBM to the modified version in a sample of knee replacement patients to assess the utility of the more specialized approach. It was expected that the more discomforting physiotherapy activities (knee bending and quadriceps exercises) would result in more pain behaviours than intermediate activities (walking and standing), which, in turn, would result in more pain behaviours than reclining. The extent to which each system reflected this expected pattern was examined.