To test the hypothesis that retaining the posterior cruciate ligament during total knee arthroplasty helps preserve the threshold of proprioceptive sensation, a machine was designed that permitted direct measurement of passive angular deflection from a resting point to the threshold of patient perception. Sixty patients with unilateral primary total knee arthroplasties were evaluated; 30 with posterior cruciate ligament retaining prostheses and 30 with posterior cruciate ligament substituting prostheses. All patients had a minimum postoperative followup of 1 year, a good or excellent result as defined by the Hospital for Special Surgery Knee Score, and no evidence of peripheral neuropathy. The gender and age distributions were equivalent between groups. The average threshold of perception for the posterior cruciate ligament retention group was 2.4 degrees. The average threshold of perception for the posterior cruciate ligament substitution group was also 2.4 degrees. Substitution or retention of the posterior cruciate ligament makes no clinical difference in proprioception as measured by threshold testing. This study provides new information for surgeons performing total knee arthroplasty to aid in the decision to retain or substitute the posterior cruciate ligament. Previous proprioception evaluation in patients with posterior cruciate ligament retaining versus posterior cruciate ligament substituting arthroplasties, using different testing methods, has revealed different results.